Geometry-based priority for the construction of candidate lists

ABSTRACT

In one example, a device includes a memory configured to store the video data; and one or more processors implemented in circuitry and configured to determine a plurality of distances between a first representative point of a current block of video data and a plurality of second representative points of neighboring blocks to the current block, add one or more of the neighboring blocks as candidates to a candidate list of the current block in an order according to the distances between the first representative point and the second representative points, and code the current block using the candidate list. The candidate list may be, for example, a merge list, an AMVP list, or a most probable mode list. Alternatively, the candidate list may be a list of candidates from which to determine context information for context-adaptive binary arithmetic coding (CABAC).

This application claims the benefit of U.S. Provisional Application No. 62/384,089, filed Sep. 6, 2016, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates to video coding.

BACKGROUND

Digital video capabilities can be incorporated into a wide range of devices, including digital televisions, digital direct broadcast systems, wireless broadcast systems, personal digital assistants (PDAs), laptop or desktop computers, tablet computers, e-book readers, digital cameras, digital recording devices, digital media players, video gaming devices, video game consoles, cellular or satellite radio telephones, so-called “smart phones,” video teleconferencing devices, video streaming devices, and the like. Digital video devices implement video coding techniques, such as those described in the standards defined by MPEG-2, MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4, Part 10, Advanced Video Coding (AVC), ITU-T H.265, also referred to as High Efficiency Video Coding (HEVC), and extensions of such standards. The video devices may transmit, receive, encode, decode, and/or store digital video information more efficiently by implementing such video coding techniques.

Video coding techniques include spatial (intra-picture) prediction and/or temporal (inter-picture) prediction to reduce or remove redundancy inherent in video sequences. For block-based video coding, a video slice (e.g., a video frame or a portion of a video frame) may be partitioned into video blocks, which for some techniques may also be referred to as treeblocks, coding units (CUs) and/or coding nodes. Video blocks in an intra-coded (I) slice of a picture are encoded using spatial prediction with respect to reference samples in neighboring blocks in the same picture. Video blocks in an inter-coded (P or B) slice of a picture may use spatial prediction with respect to reference samples in neighboring blocks in the same picture or temporal prediction with respect to reference samples in other reference pictures. Pictures may be referred to as frames, and reference pictures may be referred to a reference frames.

Spatial or temporal prediction results in a predictive block for a block to be coded. Residual data represents pixel differences between the original block to be coded and the predictive block. An inter-coded block is encoded according to a motion vector that points to a block of reference samples forming the predictive block, and the residual data indicating the difference between the coded block and the predictive block. An intra-coded block is encoded according to an intra-coding mode and the residual data. For further compression, the residual data may be transformed from the pixel domain to a transform domain, resulting in residual transform coefficients, which then may be quantized. The quantized transform coefficients, initially arranged in a two-dimensional array, may be scanned in order to produce a one-dimensional vector of transform coefficients, and entropy coding may be applied to achieve even more compression.

SUMMARY

In general, this disclosure describes techniques related to construction of candidate lists. Candidate lists may be constructed for various video coding techniques, such as signaling of intra-prediction modes, motion information coding (e.g., in merge mode or advanced motion vector prediction (AMVP) mode), or other such video coding techniques. This disclosure describes geometry-based priority for construction of candidate lists. In some aspects, geometry information, e.g., distances between the current block and the neighboring blocks, may be used to determine the priority or insertion order of candidates for the construction of candidate lists.

In one example, a method of coding video data includes determining a plurality of distances between a first representative point of a current block of video data and a plurality of second representative points of neighboring blocks to the current block, adding one or more of the neighboring blocks as candidates to a candidate list of the current block in an order according to the distances between the first representative point and the second representative points, and coding the current block using the candidate list.

In another example, a device for coding video data includes a memory configured to store the video data; and one or more processors implemented in circuitry and configured to determine a plurality of distances between a first representative point of a current block of video data and a plurality of second representative points of neighboring blocks to the current block, add one or more of the neighboring blocks as candidates to a candidate list of the current block in an order according to the distances between the first representative point and the second representative points, and code the current block using the candidate list.

In another example, a device for coding video data includes means for determining a plurality of distances between a first representative point of a current block of video data and a plurality of second representative points of neighboring blocks to the current block, means for adding one or more of the neighboring blocks as candidates to a candidate list of the current block in an order according to the distances between the first representative point and the second representative points, and means for coding the current block using the candidate list.

In another example, a computer-readable storage medium having stored thereon instructions that, when executed, cause a processor to determine a plurality of distances between a first representative point of a current block of video data and a plurality of second representative points of neighboring blocks to the current block, add one or more of the neighboring blocks as candidates to a candidate list of the current block in an order according to the distances between the first representative point and the second representative points, and code the current block using the candidate list.

The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating spatial neighboring candidates in High Efficiency Video Coding (HEVC).

FIG. 2 is a conceptual diagram illustrating temporal motion vector prediction (TMVP) in HEVC.

FIG. 3 is a conceptual diagram illustrating an example prediction structure for 3D-HEVC.

FIG. 4 is a conceptual diagram illustrating sub-PU based inter-view motion prediction in 3D-HEVC.

FIG. 5 is a conceptual diagram illustrating sub-PU motion prediction from a reference picture.

FIG. 6 is a conceptual diagram illustrating relevant pictures in ATMVP (similar to TMVP).

FIG. 7 is a flowchart showing an example method according to the techniques of this disclosure.

FIG. 8 is a conceptual diagram showing one example of a PU and neighboring blocks.

FIG. 9 is a conceptual diagram showing another example of a PU and neighboring blocks.

FIG. 10 is a conceptual diagram showing another example of a PU and neighboring blocks.

FIG. 11 is a conceptual diagram showing another example of a PU and neighboring blocks.

FIG. 12 is a conceptual diagram illustrating an example of geometric information of spatial merging candidates according to techniques of this disclosure.

FIG. 13 is a conceptual diagram illustrating an example of geometric information of spatial merging candidates according to techniques of this disclosure.

FIG. 14 is a block diagram illustrating an example video encoding and decoding system that may be configured to perform the techniques of this disclosure.

FIG. 15 is a block diagram illustrating an example of video encoder that may be configured to perform the techniques of this disclosure.

FIG. 16 is a block diagram illustrating an example of video decoder that may be configured to perform the techniques of this disclosure.

FIG. 17 is a flowchart illustrating an example method of encoding video data according to the techniques of this disclosure.

FIG. 18 is a flowchart illustrating an example method of decoding video data in accordance with the techniques of this disclosure.

DETAILED DESCRIPTION

This disclosure describes techniques that may be used to improve the construction of candidate lists and context modeling in video codecs by introducing a priority based on geometry information between the current block and the neighboring blocks to determine the priority or insertion order for the construction of candidate lists such as the merging candidate list, AMVP list and intra MPM list. Moreover, this geometry information can be used for the determination of the context for CABAC coding. The order of multiple candidates (such as merging candidate, most probable intra mode candidate) may be adaptively determined by the geometry priority. It may be used in the context of advanced video codecs, such as extensions of HEVC or the next generation of video coding standards.

Video coding standards include ITU-T H.261, ISO/IEC MPEG-1 Visual, ITU-T H.262 or ISO/IEC MPEG-2 Visual, ITU-T H.263, ISO/IEC MPEG-4 Visual and ITU-T H.264 (also known as ISO/IEC MPEG-4 AVC), including its Scalable Video Coding (SVC) and Multiview Video Coding (MVC) extensions. The latest joint draft of MVC is described in “Advanced video coding for generic audiovisual services,” ITU-T Recommendation H.264, March 2010.

In addition, there is a newly developed video coding standard, namely High Efficiency Video Coding (HEVC), developed by the Joint Collaboration Team on Video Coding (JCT-VC) of ITU-T Video Coding Experts Group (VCEG) and ISO/IEC Motion Picture Experts Group (MPEG). The latest HEVC draft specification, and referred to as HEVC WD hereinafter, is available from phenix.int-evry.fr/jct/doc_end_user/documents/14_Vienna/wg11/JCTVC-N1003-v1.zip. The HEVC standard has been finalized in G. J. Sullivan; J.-R. Ohm; W.-J. Han; T. Wiegand (December 2012). “Overview of the High Efficiency Video Coding (HEVC) Standard” (PDF). IEEE Transactions on Circuits and Systems for Video Technology (IEEE) 22 (12).

ITU-T VCEG (Q6/16) and ISO/IEC MPEG (JTC 1/SC 29/WG 11) are now studying the potential need for standardization of future video coding technology with a compression capability that significantly exceeds that of the current HEVC standard (including its current extensions and near-term extensions for screen content coding and high-dynamic-range coding). The groups are working together on this exploration activity in a joint collaboration effort known as the Joint Video Exploration Team (JVET) to evaluate compression technology designs proposed by their experts in this area. The JVET first met during 19-21 Oct. 2015. And the latest version of reference software, i.e., Joint Exploration Model 3 (JEM 3) could be downloaded from: jvet.hhi.fraunhofer.de/svn/svn_HMJEMSoftware/tags/HM-16.6-JEM-3.0/. The algorithm description for JEM3 is described in J. Chen, E. Alshina, G. J. Sullivan, J.-R. Ohm, J. Boyce “Algorithm description of Joint Exploration Test Model 3”, JVET-C1001, San Diego, March 2016.

For each block, a set of motion information can be available. The set of motion information contains motion information for forward and backward prediction directions. Here forward and backward prediction directions are two prediction directions corresponding to reference picture list 0 (RefPicList0) and reference picture list 1 (RefPicList1) of a current picture or slice. The terms “forward” and “backward” do not necessarily have a geometry meaning. Instead, they are used to distinguish which reference picture list a motion vector is based on. Forward prediction means the prediction formed based on reference list 0, while backward prediction means the prediction formed based on reference list 1. In case both reference list 0 and reference list 1 are used to form a prediction for a given block, it is called bi-directional prediction.

For a given picture or slice, if only one reference picture list is used, every block inside the picture or slice is forward predicted. If both reference picture lists are used for a given picture or slice, a block inside the picture or slice may be forward predicted, or backward predicted, or bi-directionally predicted.

For each prediction direction, the motion information contains a reference index and a motion vector. A reference index is used to identify a reference picture in the corresponding reference picture list (e.g., RefPicList0 or RefPicList1). A motion vector has both a horizontal and a vertical component, with each indicating an offset value along horizontal and vertical direction respectively. In some descriptions, for simplicity, the word of “motion vector” may be used interchangeably with motion information, to indicate both the motion vector and its associated reference index.

Picture order count (POC) is widely used in video coding standards to identify a display order of a picture. Although there are cases two pictures within one coded video sequence may have the same POC value, it typically doesn't happen within a coded video sequence. When multiple coded video sequences are present in a bitstream, pictures with a same value of POC may be closer to each other in terms of decoding order.

POC values of pictures are typically used for reference picture list construction, derivation of reference picture set as in HEVC and motion vector scaling.

In Wiegand, Thomas; Sullivan, Gary J.; Bjøntegaard, Gisle; Luthra, Ajay (July 2003), “Overview of the H.264/AVC Video Coding Standard” (PDF). IEEE Transactions on Circuits and Systems for Video Technology 13 (7), H.264/AVC (Advanced Video Coding), each inter macroblock (MB) may be partitioned into four different ways:

-   -   One 16×16 MB partition     -   Two 16×8 MB partitions     -   Two 8×16 MB partitions     -   Four 8×8 MB partitions

Different MB partitions in one MB may have different reference index values for each direction (RefPicList0 or RefPicList1). When an MB is not partitioned into four 8×8 MB partitions, it has only one motion vector for each MB partition in each direction. When an MB is partitioned into four 8×8 MB partitions, each 8×8 MB partition can be further partitioned into sub-blocks, each of which can have a different motion vector in each direction. There are four different ways to get sub-blocks from an 8×8 MB partition:

-   -   One 8×8 sub-block     -   Two 8×4 sub-blocks     -   Two 4×8 sub-blocks     -   Four 4×4 sub-blocks

Each sub-block can have a different motion vector in each direction. Therefore, motion vector is present in a level equal to higher than sub-block.

In AVC, temporal direct mode could be enabled in either MB or MB partition level for skip or direct mode in B slices. For each MB partition, the motion vectors of the block co-located with the current MB partition in the RefPicList1[0] of the current block are used to derive the motion vectors. Each motion vector in the co-located block is scaled based on POC distances.

In AVC, a spatial direct mode can also be used to predict motion information from the spatial neighbors.

In HEVC, the largest coding unit in a slice is called a coding tree block (CTB) or coding tree unit (CTU). A CTB contains a quad-tree, the nodes of which are coding units (CUs).

The size of a CTB can be ranges from 16×16 to 64×64 in the HEVC main profile (although technically 8×8 CTB sizes can be supported). A coding unit (CU) could be the same size of a CTB although and as small as 8×8. Each coding unit is coded with one mode. When a CU is inter coded, it may be further partitioned into 2 or 4 prediction units (PUs) or become just one PU when further partition doesn't apply. When two PUs are present in one CU, they can be half size rectangles or two rectangle size with ¼ or ¾ size of the CU.

When the CU is inter coded, one set of motion information is present for each PU. In addition, each PU is coded with a unique inter-prediction mode to derive the set of motion information.

In the HEVC standard, there are two inter prediction modes, named merge (skip is considered as a special case of merge) and advanced motion vector prediction (AMVP) modes respectively for a prediction unit (PU).

In either AMVP or merge mode, a motion vector (MV) candidate list is maintained for multiple motion vector predictors. The motion vector(s), as well as reference indices in the merge mode, of the current PU are generated by taking one candidate from the MV candidate list.

The MV candidate list contains up to 5 candidates for the merge mode and only two candidates for the AMVP mode. A merge candidate may contain a set of motion information, e.g., motion vectors corresponding to both reference picture lists (list 0 and list 1) and the reference indices. If a merge candidate is identified by a merge index, the reference pictures are used for the prediction of the current blocks, as well as the associated motion vectors are determined. However, under AMVP mode for each potential prediction direction from either list 0 or list 1, a reference index needs to be explicitly signaled, together with an MVP index to the MV candidate list since the AMVP candidate contains only a motion vector. In AMVP mode, the predicted motion vectors can be further refined.

As can be seen above, a merge candidate corresponds to a full set of motion information while an AMVP candidate contains just one motion vector for a specific prediction direction and reference index. That is, in general, motion information includes a motion vector predictor, a reference picture list, an index into the reference picture list, and in the case of AMVP, differences to be applied to the motion vector predictor. According to HEVC, in merge mode, the motion vector, reference picture list, and index are inherited from a selected candidate, whereas in AMVP, the motion vector predictor corresponds to the motion vector of a selected candidate, and the reference picture list, index, and motion vector difference values are signaled.

The candidates for both modes are derived similarly from the same spatial and temporal neighboring blocks.

FIG. 1 is a conceptual diagram illustrating spatial neighboring candidates in HEVC. Spatial MV candidates are derived from the neighboring blocks shown on FIG. 1, for a specific PU (PU₀), although the methods generating the candidates from the blocks differ for merge and AMVP modes.

In merge mode, up to four spatial MV candidates can be derived with the orders showed on FIG. 1 (a) with numbers, and the order is the following: left (0, A1), above (1, B1), above right (2, B0), below left (3, A0), and above left (4, B2), as shown in FIG. 1 (a).

In merge mode, up to four spatial MV candidates can be derived with the orders shown in FIG. 1(a) with numbers, and the order is the following: left (0, A1), above (1, B1), above-right (2, B0), below-left (3, A0), and above left (4, B2), as shown in FIG. 1 (a). That is, in FIG. 4(a), block 100 includes PU0 104A and PU1 104B. When a video coder (e.g., a video encoder or a video decoder) is to code (encode or decode) motion information for PU0 104A using merge mode, the video coder adds motion information from spatial neighboring blocks 108A, 108B, 108C, 108D, and 108E to a candidate list, in that order. Blocks 108A, 108B, 108C, 108D, and 108E may also be referred to as, respectively, blocks A1, B1, B0, A0, and B2, as in HEVC.

In AVMP mode, the neighboring blocks are divided into two groups: a left group including blocks 0 and 1, and an above group including blocks 2, 3, and 4 as shown on FIG. 1 (b). These blocks are labeled, respectively, as blocks 110A, 110B, 110C, 110D, and 110E in FIG. 1(b). In particular, in FIG. 1(b), block 102 includes PU0 106A and PU1 106B, and blocks 110A, 110B, 110C, 110D, and 110E represent spatial neighbors to PU0 106A. For each group, the potential candidate in a neighboring block referring to the same reference picture as that indicated by the signaled reference index has the highest priority to be chosen to form a final candidate of the group. It is possible that all neighboring blocks do not contain a motion vector pointing to the same reference picture. Therefore, if such a candidate cannot be found, the first available candidate will be scaled to form the final candidate; thus, the temporal distance differences can be compensated.

FIG. 2 is a conceptual diagram illustrating temporal motion vector prediction in HEVC. In particular, FIG. 2(a) illustrates an example CU 120 including PU0 122A and PU 1 122B. PU0 122A includes a center block 126 for PU 122A and a bottom-right block 124 to PU0 122A. FIG. 2(a) also shows an external block 128 for which motion information may be predicted from motion information of PU0 122A, as discussed below. FIG. 2(b) illustrates a current picture 130 including a current block 138 for which motion information is to be predicted. In particular, FIG. 2(b) illustrates a collocated picture 134 to current picture 130 (including collocated block 140 to current block 138), a current reference picture 132, and a collocated reference picture 136. Collocated block 140 is predicted using motion vector 144, which is used as a temporal motion vector predictor (TMVP) 142 for motion information of block 138.

A video coder may add a TMVP candidate (e.g., TMVP candidate 142) into the MV candidate list after any spatial motion vector candidates if TMVP is enabled and the TMVP candidate is available. The process of motion vector derivation for the TMVP candidate is the same for both merge and AMVP modes. However, the target reference index for the TMVP candidate in the merge mode is set to 0, according to HEVC.

The primary block location for the TMVP candidate derivation is the bottom right block outside of the collocated PU, as shown in FIG. 2 (a) as block 124 to PU0 122A, to compensate the bias to the above and left blocks used to generate spatial neighboring candidates. However, if block 124 is located outside of the current CTB row or motion information is not available for block 124, the block is substituted with center block 126 of the PU as shown in FIG. 2(a).

The motion vector for TMVP candidate 142 is derived from co-located block 140 of co-located picture 134, as indicated in slice level information. The motion vector for the co-located PU is called collocated MV.

Similar to temporal direct mode in AVC, a motion vector of the TMVP candidate may be subject to motion vector scaling, which is performed to compensate picture order count (POC) distance differences and/or temporal distance distances between current picture 130 and current reference picture 132, and collocated picture 134 and collocated reference picture 136. That is, motion vector 144 may be scaled to produce TMVP candidate 142, based on these POC/temporal distance differences.

Several aspects of merge and AMVP modes are worth mentioning as follows.

Motion vector scaling: It is assumed that the value of motion vectors is proportional to the distance of pictures in the presentation time. A motion vector associates two pictures, the reference picture, and the picture containing the motion vector (namely the containing picture). When a motion vector is utilized to predict the other motion vector, the distance of the containing picture and the reference picture is calculated based on the Picture Order Count (POC) values.

For a motion vector to be predicted, both its associated containing picture and reference picture may be different. Therefore a new distance (based on POC) is calculated. And the motion vector is scaled based on these two POC distances. For a spatial neighboring candidate, the containing pictures for the two motion vectors are the same, while the reference pictures are different. In HEVC, motion vector scaling applies to both TMVP and AMVP for spatial and temporal neighboring candidates.

Artificial motion vector candidate generation: If a motion vector candidate list is not complete, artificial motion vector candidates are generated and inserted at the end of the list until it will have all candidates.

In merge mode, there are two types of artificial MV candidates: combined candidate derived only for B-slices and zero candidates used only for AMVP if the first type doesn't provide enough artificial candidates.

For each pair of candidates that are already in the candidate list and have necessary motion information, bi-directional combined motion vector candidates are derived by a combination of the motion vector of the first candidate referring to a picture in the list 0 and the motion vector of a second candidate referring to a picture in the list 1.

Pruning process for candidate insertion: Candidates from different blocks may happen to be the same, which decreases the efficiency of a merge/AMVP candidate list. A pruning process is applied to solve this problem. It compares one candidate against the others in the current candidate list to avoid inserting identical candidate in certain extent. To reduce the complexity, only limited numbers of pruning process is applied instead of comparing each potential one with all the other existing ones.

FIG. 3 illustrates an example prediction structure for 3D-HEVC. 3D-HEVC is a 3D video extension of HEVC under development by JCT-3V. 3D-HEVC is described in Gerhard Tech; Krzysztof Wegner; Ying Chen; Sehoon Yea (Feb. 18, 2015). “3D-HEVC Draft Text 7,” JCT-3V. Certain techniques related to the techniques of this disclosure are described with respect to FIGS. 3 and 4 below.

FIG. 3 shows a multiview prediction structure for a three-view case. V3 denotes the base view and a picture in a non-base view (V1 or V5) can be predicted from pictures in a dependent (base) view of the same time instance.

It is worth mentioning that the inter-view sample prediction (from reconstructed samples) is supported in MV-HEVC, a typical prediction structure of which is shown in FIG. 3.

Both MV-HEVC and 3D-HEVC are compatible to HEVC in a way that the base (texture) view is decodable by HEVC (version 1) decoder.

In MV-HEVC, a current picture in a non-base view may be predicted by both pictures in the same view and pictures in a reference view of the same time instance, by putting all of these pictures in reference picture lists of the picture. Therefore, a reference picture list of the current picture contains both temporal reference pictures and inter-view reference pictures.

A motion vector associated with a reference index corresponding to a temporal reference picture is denoted as a temporal motion vector.

A motion vector associated with a reference index corresponding to an inter-view reference picture is denoted as a disparity motion vector.

3D-HEVC supports all features in MV-HEVC. Therefore, inter-view sample prediction as mentioned above is enabled.

In addition, more advanced texture only coding tools and depth related/dependent coding tools are supported.

The texture-only coding tools often require the identification of the corresponding blocks (between views) that may belong to the same object. Therefore, disparity vector derivation is a basic technology in 3D-HEVC.

Inter-view sample prediction (from reconstructed samples) is supported in MV-HEVC, a typical prediction structure of which is shown in FIG. 5.

FIG. 4 is a conceptual diagram illustrating sub-PU based inter-view motion prediction in 3D-HEVC. FIG. 4 shows current picture 160 of a current view (V1) and a collocated picture 162 in a reference view (V0). Current picture 160 includes a current PU 164 including four sub-Pus 166A-166D (sub-PUs 166). Respective disparity vectors 174A-174D (disparity vectors 174) identify corresponding sub-PUs 168A-168D to sub-PUs 166 in collocated picture 162. In 3D-HEVC, a sub-PU level inter-view motion prediction method for the inter-view merge candidate, i.e., the candidate derived from a reference block in the reference view.

When such a mode is enabled, current PU 164 may correspond to a reference area (with the same size as current PU identified by the disparity vector) in the reference view and the reference area may have richer motion information than needed for generation one set of motion information typically for a PU. Therefore, a sub-PU level inter-view motion prediction (SPIVMP) method may be used, as shown in FIG. 4.

This mode may also be signaled as a special merge candidate. Each of the sub-PUs contains a full set of motion information. Therefore, a PU may contain multiple sets of motion information.

Similarly, in depth coding of 3D-HEVC, it is designed that the Motion Parameter Inheritance (MPI) candidate derived from texture view can also be extended in a way similar to sub-PU level inter-view motion prediction.

For example, if the current depth PU has a co-located region which contains multiple PUs, the current depth PU may be separated into sub-PUs, each may have a different set of motion information.

This method is called sub-PU MPI.

Example sub-PU related techniques for 2D video coding are described in U.S. application Ser. No. 14/497,128, the entirety of which is incorporated by reference herein. In U.S. application Ser. No. 14/497,128, a sub-PU based advanced TMVP (ATMVP) design has been proposed.

In single-layer coding, a two-stage advanced temporal motion vector prediction design is proposed. The first stage is utilized to derive a vector identifying the corresponding block of the current prediction unit (PU) in a reference picture and a second stage is to extract multiple sets motion information from the corresponding block and assign them to sub-PUs of the PU. Each sub-PU of the PU therefore is motion compensated separately. The concept of the ATMVP is summarized as follows: (1) The vector in the first stage can be derived from spatial and temporal neighboring blocks of the current PU. (2) This process may be achieved as activating a merge candidate among all the other merge candidates.

Applicable to single-layer coding and sub-PU temporal motion vector prediction, a PU or CU may have motion refinement data to be conveyed on top of the predictors.

Several design aspects of U.S. application Ser. No. 14/497,128 are highlighted as follows:

-   1. The first stage of vector derivation can also be simplified by     just a zero vector. -   2. The first stage of vector derivation may include identifying     jointly the motion vector and its associated picture. Various ways     of selecting the associated picture and further deciding the motion     vector to be the first stage vector have been proposed. -   3. If the motion information during the above process is     unavailable, the “first stage vector” is used for substitution. -   4. A motion vector identified from a temporal neighbor has to be     scaled to be used for the current sub-PU, in a way similar to motion     vector scaling in TMVP. However, which reference picture such a     motion vector may be scaled to can be designed with one of the     following ways:     -   a. The picture is identified by a fixed reference index of the         current picture.     -   b. The picture is identified to be the reference picture of the         corresponding temporal neighbor, if also available in a         reference picture list of the current picture.     -   c. The picture is set to be the co-located picture identified in         the first stage and from where the motion vectors are grabbed         from.

To address some design issues in U.S. application Ser. No. 14/497,128, the following techniques were proposed in U.S. application Ser. No. 15/005,564, the entire content of which is incorporated by reference herein:

-   -   1. Position of the ATMVP candidate, if inserted, e.g., as a         merge candidate list         -   a. Assume the spatial candidates and TMVP candidate are             inserted into a merge candidate list in a certain order. The             ATMVP candidate may be inserted in any relatively fixed             position of those candidates.             -   i. In one alternative, for example, the ATMVP candidate                 can be inserted in the merge candidate list after the                 first two spatial candidates e.g., A1 and B1;             -   ii. In one alternative, for example, the ATMVP candidate                 can be inserted after the first three spatial candidates                 e.g., A1 and B1 and B0;             -   iii. In one alternative, for example, the ATMVP                 candidate can be inserted after the first four                 candidates e.g., A1, B1, B0, and A0.             -   iv. In one alternative, for example, the ATMVP candidate                 can be inserted right before the TMVP candidate.             -   v. In one alternatively, for example, the ATMVP                 candidate can be inserted right after the TMVP                 candidate.         -   b. Alternatively, the position of ATMVP candidate in the             candidate list can be signaled in the bitstream. The             positions of other candidates, including the TMVP candidate             can be additionally signaled.     -   2. Availability check of the ATMVP candidate can apply by         accessing just one set of motion information. When such set of         information is unavailable, e.g., one block being intra-coded,         the whole ATMVP candidate is considered as unavailable. In that         case, the ATMVP will not be inserted into the merge list.         -   a. A center position or a center sub-PU is used purely to             check the availability of the ATMVP candidate. When a center             sub-PU is used, the center sub-PU is chosen to be the one             that covers the center position (e.g., the center 3             position, with a relative coordinate of (W/2, H/2) to the             top-left sample of the PU, wherein W×H is the size of the             PU). Such a position or center sub-PU may be used together             with the temporal vector to identify a corresponding block             in the motion source picture. A set of motion information             from the block that covers the center position of a             corresponding block is identified.     -   3. Representative set of motion information for the ATMVP coded         PU from a sub-PU.         -   a. To form the ATMVP candidate the representative set of             motion information is first formed.         -   b. Such a representative set of motion information may be             derived from a fixed position or fixed sub-PU. It can be             chosen in the same way as that of the set of motion             information used to determine the availability of the ATMVP             candidate, as described in bullet #2.         -   c. When a sub-PU has identified its own set of motion             information and is unavailable, it is set to be equal to the             representative set of motion information.         -   d. If the representative set of motion information is set to             be that of a sub-PU, no additional motion storage is needed             at the decoder side for the current CTU or slice in the             worst case scenario.         -   e. Such a representative set of motion information is used             in all scenarios when the decoding processes requires the             whole PU to be represented by one set of motion information,             including pruning, such that the process is used to generate             combined bi-predictive merging candidates.     -   4. The ATMVP candidate is pruned with TMVP candidate and         interactions between TMVP and ATMVP can be considered; detailed         techniques are listed below:         -   a. The pruning of a sub-PU based candidate, e.g., ATMVP             candidate with a normal candidate, may be conducted by using             the representative set of motion information (as in bullet             #3) for such a sub-PU based candidate. If such set of motion             information is the same as a normal merge candidate, the two             candidates are considered as the same.         -   b. Alternatively, in addition, a check is performed to             determine whether the ATMVP contains multiple different sets             of motion information for multiple sub-Pus; if at least two             different sets are identified, the sub-PU based candidate is             not used for pruning, i.e., is considered to be different to             any other candidate; Otherwise, it may be used for pruning             (e.g., may be pruned during the pruning process).         -   c. Alternatively, in addition, the ATMVP candidate may be             pruned with the spatial candidates, e.g., the left and top             ones only, with positions denoted as A1 and B1.         -   d. Alternatively, only one candidate is formed from temporal             reference, being either ATMVP candidate or TMVP candidate.             When ATMVP is available, the candidate is ATMVP; otherwise,             the candidate is TMVP. Such a candidate is inserted into the             merge candidate list in a position similar to the position             of TMVP. In this case, the maximum number of candidates may             be kept as unchanged.             -   i. Alternatively, TMVP is always disabled even when                 ATMVP is unavailable.             -   ii. Alternatively, TMVP is used only when ATMVP is                 unavailable.         -   e. Alternatively, when ATMVP is available and TMVP is             unavailable, one set of motion information of one sub-PU is             used as the TMVP candidate. In this case, furthermore, the             pruning process between ATMVP and TMVP is not applied.         -   f. Alternatively, or additionally, the temporal vector used             for ATMVP may be also used for TMVP, such that the             bottom-right position or center 3 position as used for             current TMVP in HEVC do not need to be used.             -   i. Alternatively, the position identified by the                 temporal vector and the bottom-right and center 3                 positions are jointly considered to provide an available                 TMVP candidate.     -   5. Multiple availability checks for ATMVP are supported to give         higher chances for the ATMVP candidate to be more accurate and         efficient. When the current ATMVP candidate from the motion         source picture as identified by the first temporal vector (e.g.,         as shown in FIG. 9) is unavailable, other pictures can be         considered as motion source picture. When another picture is         considered, it may be associated with a different second         temporal vector, or may be associated simply with a second         temporal vector scaled from the first temporal vector that         points to the unavailable ATMVP candidate.         -   a. A second temporal vector can identify an ATMVP candidate             in a second motion source picture and the same availability             check can apply. If the ATMVP candidate as derived from the             second motion source picture is available, the ATMVP             candidate is derived and no other pictures need to be             checked; otherwise, other pictures as motion source pictures             need to be checked.         -   b. Pictures to be checked may be those in the reference             picture lists of the current picture, with a given order.             For each list, the pictures are checked in the ascending             order of the reference index. List X is first checked and             pictures in list Y (being 1−X) follows.             -   i. List X is chosen so that list X is the list that                 contains the co-located picture used for TMVP.             -   ii. Alternatively, X is simply set to be 1 or 0.         -   c. Pictures to be checked are those identified by motion             vectors of the spatial neighbors, with a given order.     -   6. A partition of the PU that the current ATMVP apply to may be         2N×2N, N×N, 2N×N, N×2N or asymmetric motion partition (AMP)         partitions, such as 2N×N/2.         -   a. Alternatively, in addition, if other partition sizes can             be allowed, ATMVP can be supported too, and such a size may             include e.g., 64×8.         -   b. Alternatively, the mode may be only applied to certain             partitions, e.g., 2N×2N.     -   7. The ATMVP candidate is marked as a different type of merge         candidate.     -   8. When identifying a vector (temporal vector as in the first         stage) from neighbors, multiple neighboring positions, e.g.,         those used in merge candidate list construction, can be checked         in order. For each of the neighbors, the motion vectors         corresponding to reference picture list 0 (list 0) or reference         picture list 1 (list 1) can be checked in order. When two motion         vectors are available, the motion vectors in list X can be         checked first and followed by list Y (with Y being equal to         1−X), so that list X is the list that contains the co-located         picture used for TMVP. In ATMVP, a temporal vector is used be         added as a shift of any center position of a sub-PU, wherein the         components of temporal vector may need to be shifted to integer         numbers. Such a shifted center position is used to identify a         smallest unit that motion vectors can be allocated to, e.g.,         with a size of 4×4 that covers the current center position.         -   a. Alternatively, motion vectors corresponding to list 0 may             be checked before those corresponding to list 1;         -   b. Alternatively, motion vectors corresponding to list 1 may             be checked before those corresponding to list 0;         -   c. Alternatively, all motion vectors corresponding to list X             in all spatial neighbors are checked in order, followed by             the motion vectors corresponding to list Y (with Y being             equal to 1−X). Here, list “X” can be the list that indicates             where co-located picture belongs, or just simply set to be 0             or 1.         -   d. The order of the spatial neighbors can be the same as             that used in HEVC merge mode.     -   9. When in the first stage of identifying a temporal vector does         not include information identifying a reference picture, the         motion source picture as shown in FIG. 9, may be simply set to         be a fixed picture, e.g., the co-located picture used for TMVP.         -   a. In such a case, the vector may only be identified from             the motion vectors that point to such a fixed picture.         -   b. In such a case, the vector may only be identified from             the motion vectors that point to any picture but further             scaled towards the fixed picture.     -   10. When in the first stage of identifying a vector consists         identifying a reference picture, the motion source picture as         shown in FIG. 9, one or more of the following additional checks         may apply for a candidate motion vector.         -   a. If the motion vector is associated with a picture or             slice that is Intra coded, such a motion vector is             considered as unavailable and cannot be used to be converted             to the vector.         -   b. If the motion vector identifies an Intra block (by e.g.,             adding the current center coordinate with the motion vector)             in the associated picture, such a motion vector is             considered as unavailable and cannot be used to be converted             to the vector.     -   11. When in the first stage of identifying a vector, the         components of the vector may be set to be (half width of the         current PU, half height of the current PU), so that it         identifies a bottom-right pixel position in the motion source         picture. Here (x, y) indicates a horizontal and vertical         components of one motion vector.         -   a. Alternatively, the components of the vector may be set to             be (sum(half width of the current PU, M), sum(half height of             the current PU, N)) where the function sum(a, b) returns the             sum of a and b. In one example, when the motion information             is stored in 4×4 unit, M and N are both set to be equal             to 2. In another example, when the motion information is             stored in 8×8 unit, M and N are both set to be equal to 4.     -   12. The sub-block/sub-PU size when ATMVP applies is signaled in         a parameter set, e.g., sequence parameter set of picture         parameter set. The size ranges from the least PU size to the CTU         size. The size can be also pre-defined or signaled. The size can         be, e.g., as small as 4×4. Alternatively, the sub-block/sub-PU         size can be derived based on the size of PU or CU. For example,         the sub-block/sub-PU can be set equal to max (4×4, (width of         CU)»M). The value of M can be pre-defined or signaled in the         bitstream.     -   13. The maximum number of merge candidates may be increased by 1         due to the fact that ATMVP can be considered as a new merge         candidate. For example, compared to HEVC which takes up to 5         candidates in a merge candidate list after pruning, the maximum         number of merge candidates can be increased to 6.         -   a. Alternatively, pruning with conventional TMVP candidate             or unification with the conventional TMVP candidate can be             performed for ATMVP such that the maximum number of merge             candidates can be kept as unchanged.         -   b. Alternatively, when ATMVP is identified to be available,             a spatial neighboring candidate is excluded from the merge             candidate list, e.g., the last spatial neighboring candidate             in fetching order is excluded.     -   14. When multiple spatial neighboring motion vectors are         considered to derive the temporal vector, a motion vector         similarity may be calculated based on the neighboring motion         vectors of the current PU as well as the neighboring motion         vectors identified by a specific temporal vector being set equal         to a motion vector. The one that leads to the highest motion         similarity may be chosen as the final temporal vector.         -   a. In one alternative, for each motion vector from a             neighboring position N, the motion vector identifies a block             (same size as the current PU) in the motion source picture,             wherein its neighboring position N contains a set of the             motion information. This set of motion vector is compared             with the set of motion information as in the neighboring             position N of the current block.         -   b. In another alternative, for each motion vector from a             neighboring position N, it identifies a block in the motion             source picture, wherein its neighboring positions contain             multiple sets of motion information. These multiple sets of             motion vector are compared with the multiple sets of motion             information from the neighboring positions of the current PU             in the same relative positions. A motion information             similarity is calculated. For example, the current PU has             the following sets of motion information from A1, B1, A0 and             B0, denoted as MI_(A1), MI_(B1), MI_(A0) and MI_(B0). For a             temporal vector TV, it identifies a block corresponding to             the PU in the motion source picture. Such a block has motion             information from the same relative A1, B1, A0 and B0             positions, and denoted as TMI_(A1), TMI_(B1), TMI_(A0) and             TMI_(B0). The motion similarity as determined by TV is             calculated as MS_(tv)=Σ_(N∈{A1,B1,A0,B0})MVSim(MI_(N),             TMI_(N)), wherein MVSim defines the similarity between two             sets of motion information.         -   c. In both of the above cases, the motion similarity MVSim             can be used, wherein the two input parameters are the two             motion information, each contains up to two motion vectors             and two reference indices. Since each pair of the motion             vectors in list X are actually associated with reference             pictures in different list X of different pictures, the             current picture and the motion source picture. For each of             the two motion vectors MVX_(N) and TMVX_(N) (with X being             equal to 0 or 1), the motion vector difference MVDX_(N) can             be calculated as MVX_(N)−TMVX_(N). Afterwards, the             difference MVSimX is calculated as e.g.,             abs(MVDX_(N)[0])+abs(MVDX_(N)[1]), or             (MVDX_(N)[0]*MVDX_(N)[0]+MVDX_(N)[1]*MVDX_(N)[1]). If both             sets of motion information contains available motion             vectors, MVSim is set equal to MVSim0+MVSim1.             -   i. In order to have a unified calculation of the motion                 difference, both of the motion vectors need to be scaled                 towards the same fixed picture, which can be, e.g., the                 first reference picture RefPicListX[0] of the list X of                 the current picture.             -   ii. If the availability of the motion vector in list X                 from the first set and the availability of the motion                 vector in list X from the second set are different,                 i.e., one reference index is −1 while the other is not,                 such two sets of motion information are considered as                 not similar in direction X. If the two sets are not                 similar in both sets, the final MVSim function may                 return a big value T, which may be, e.g., considered as                 infinite.             -   iii. Alternatively, for a pair of sets of motion                 information, if one is predicted from list X (X being                 equal to 0 or 1) but not list Y (Y being equal to 1−X)                 and the other has the same status, a weighting between 1                 and 2 (e.g., MVSim is equal to MVSimX*1.5) may be used.                 When one set is only predicted from list X and the other                 is only predicted from list Y, MVSim is set to the big                 value T.             -   iv. Alternatively, for any set of motion information, as                 long as one motion vector is available, both motion                 vectors will be produced. In the case that only one                 motion vector is available (corresponding to list X), it                 is scaled to form the motion vector corresponding to the                 other list Y.         -   d. Alternatively, the motion vector may be measured based on             differences between the neighboring pixels of the current PU             and the neighboring pixels of the block (same size as the             current PU) identified by the motion vector. The motion             vector that leads to the smallest difference may be chosen             as the final temporal vector.     -   15. When deriving the temporal vector of the current block,         motion vectors and/or temporal vectors from neighboring blocks         that are coded with ATMVP may have a higher priority than motion         vectors from other neighboring blocks.         -   a. In one example, only temporal vectors of neighboring             blocks are checked first, and the first available one can be             set to the temporal vector of the current block. Only when             such temporal vectors are not present, normal motion vectors             are further checked. In this case, temporal vectors for             ATMVP coded blocks need to be stored.         -   b. In another example, only motion vectors from ATMVP coded             neighboring blocks are checked first, and the first             available one can be set to the temporal vector of the             current block. Only when such temporal vectors are not             present, normal motion vectors are further checked.         -   c. In another example, only motion vectors from ATMVP coded             neighboring blocks are checked first, and the first             available one can be set to the temporal vector of the             current block. If such motion vectors are not available, the             checking of temporal vector continues similar as in bullet             15 a.         -   d. In another example, temporal vectors from neighboring             blocks are checked first, the first available one can be set             to the temporal vector of the current block. If such motion             vectors are not available, the checking of temporal vector             continues similar as in bullet 15 b.         -   e. In another example, temporal vectors and motion vectors             of ATMVP coded neighboring blocks are checked first, the             first available one can be set to the temporal vector of the             current block. Only when such temporal vectors and motion             vectors are not present, normal motion vectors are further             checked.     -   16. When multiple spatial neighboring motion vectors are         considered to derive the temporal vector, a motion vector may be         chosen so that it minimizes the distortion that is calculated         from the pixel domain, e.g., template matching may be used to         derive the temporal vector such that the one leads to minimal         matching cost is selected as the final temporal vector.     -   17. Derivation of a set of motion information from a         corresponding block (in the motion source picture) is done in a         way that when a motion vector is available in the corresponding         block for any list X (denote the motion vector to be MVX), for         the current sub-PU of the ATMVP candidate, the motion vector is         considered as available for list X (by scaling the MVX). If the         motion vector is unavailable in the corresponding block for any         list X, the motion vector is considered as unavailable for list         X.         -   a. Alternatively, when motion vector in the corresponding             block is unavailable for list X but available for list 1-X             (denoted 1-X by Y and denote the motion vector to be MVY),             the motion vector is still considered as available for list             X (by scaling the MVY towards the target reference picture             in list X).         -   b. Alternatively, or in addition, when both motion vectors             in the corresponding block for list X and list Y (equal to             1-X) are available, the motion vectors from list X and list             Y are not necessary used to directly scale and generate the             two motion vectors of a current sub-PU by scaling.             -   i. In one example, when formulating the ATMVP candidate,                 the low-delay check as done in TMVP applies to each                 sub-PU. If for every picture (denoted by refPic) in                 every reference picture list of the current slice,                 picture order count (POC) value of refPic is smaller                 than POC of current slice, current slice is considered                 with low-delay mode. In this low-delay mode, motion                 vectors from list X and list Y are scaled to generate                 the motion vectors of a current sub-PU for list X and                 list Y, respectively. When not in the low-delay mode,                 only one motion vector MVZ from MVX or MVY is chosen and                 scaled to generate the two motion vectors for a current                 sub-PU. Similar to TMVP, in such a case Z is set equal                 to collocated_from_10_flag, meaning that it depends on                 whether the co-located picture as in TMVP is in the list                 X or list Y of the current picture. Alternatively, Z is                 set as follows: if the motion source picture is                 identified from list X, Z is set to X. Alternatively, in                 addition, when the motion source pictures belong to both                 reference picture lists, and RefPicList0[idx0] is the                 motion source picture that is first present in list 0                 and RefPicList(1)[idx1] is the motion source picture                 that is first present in list 1, Z is set to be 0 if                 idx0 is smaller than or equal to idx1, and set to be 1                 otherwise.     -   18. The motion source picture may be signaled, e.g., generated         by video encoder 20 in a coded bitstream. In detail, a flag         indicating whether the motion source picture is from list 0 or         list 1 is signaled for a B slice. Alternatively, in addition, a         reference index to a list 0 or list 1 of the current picture may         be signaled to identify the motion source picture.

When identifying a temporal vector, a vector is considered as unavailable (thus other ones can be considered) if it points to an Intra coded block in the associated motion source picture.

FIG. 5 is a conceptual diagram illustrating sub-PU motion prediction from a reference picture. In this example, current picture 180 includes a current PU 184 (e.g., a PU). In this example, motion vector 192 identifies PU 186 of reference picture 182 relative to PU 184. PU 186 is partitioned into sub-PUs 188A-188D, each having respective motion vectors 190A-190D. Thus, although current PU 184 is not actually partitioned into separate sub-PUs, in this example, current PU 184 may be predicted using motion information from sub-PUs 188A-188D. In particular, a video coder may code sub-PUs of current PU 184 using respective motion vectors 190A-190D. However, the video coder need not code syntax elements indicating that current PU 184 is split into sub-PUs. In this manner, current PU 184 may be effectively predicted using multiple motion vectors 190A-190D, inherited from respective sub-PUs 188A-188D, without the signaling overhead of syntax elements used to split current PU 184 into multiple sub-PUs.

FIG. 6 is a conceptual diagram illustrating relevant pictures in ATMVP (similar to TMVP). In particular, FIG. 9 illustrates current picture 204, motion source picture 206, and reference pictures 200, 202. More particularly, current picture 204 includes current block 208. Temporal motion vector 212 identifies corresponding block 210 of motion source picture 206 relative to current block 208. Corresponding block 210, in turn, includes motion vector 214, which refers to reference picture 202 and acts as an advanced temporal motion vector predictor for at least a portion of current block 208, e.g., a sub-PU of current block 208. That is, motion vector 214 may be added as a candidate motion vector predictor for current block 208. If selected, at least a portion of current block 208 may be predicted using a corresponding motion vector, namely, motion vector 216, which refers to reference picture 200.

Sub-PU related techniques for HEVC are also described in U.S. Application Nos. 62/174,393 and 62/295,329, the entire content of both of which is incorporated by reference herein. A flowchart showing an example technique for spatial-temporal motion vector predictor derivation is shown below in FIG. 7.

To enhance the performance using sub-PU motion prediction, spatial-temporal motion information of neighboring sub-PU's (ATMVP_EXT) are exploited as described in U.S. Application Nos. 62/174,393 and 62/295,329. In this example, the motion vector for each sub-PU is derived from the information of neighboring blocks in three-dimensional domain. It means the neighboring blocks could be the spatial neighbors in the current picture or the temporal neighbors in previous coded pictures. FIG. 7 shows the flow chart of the spatial-temporal motion vector predictor (STMVP) derivation process. Besides what are described below, the methods described above for ATMVP (e.g., bullet #1, #2, #3, #4, #6, #7, #12, #13) could be directly extended to STMVP.

The method of FIG. 7 may be performed by video encoder 20 and/or video decoder 30 (as described in more detail below). For generality, the method of FIG. 7 is explained as being performed by a “video coder,” which again, may correspond to either of video encoder 20 or video decoder 30.

Initially, a video coder obtains an available motion field from spatial or temporal neighboring blocks for a current sub-PU of a PU (230). The video coder then derives motion information from the obtained neighboring motion field (232). The video coder then determines whether motion information for all sub-PUs of the PU has been derived (234). If not (“NO” branch of 234), the video coder derives motion information for a remaining sub-PU (230). On the other hand, if motion information for all sub-PUs has been derived (“YES” branch of 234), the video coder determines availability of a spatial-temporal sub-PU motion predictor (236), e.g., as explained above. The video coder inserts the spatial-temporal sub-PU motion predictor into a merge list, if the spatial-temporal sub-PU motion predictor is available (238).

Although not shown in the method of FIG. 7, the video coder may then code the PU (e.g., each of the sub-PUs of the PU) using the merge candidate list. For instance, when performed by video encoder 20, video encoder 20 may calculate residual block(s) for the PU (e.g., for each sub-PU) using the sub-PUs as predictors, transform and quantize the residual block(s), and entropy encode the resulting quantized transform coefficients. Video decoder 30, similarly, may entropy decode received data to reproduce quantized transform coefficients, inverse quantize and inverse transform these coefficients to reproduce the residual block(s), and then combine the residual block(s) with the corresponding sub-PUs to decode a block corresponding to the PU.

In the following description, the term “block” is used to refer the block-unit for storage of prediction related info, e.g., inter or intra prediction, intra prediction mode, motion information etc. Such prediction info is saved and may be used for coding future blocks, e.g., predicting the prediction mode information for future blocks. In AVC and HEVC, the size of such a block is 4×4. It is noted that in the following description, ‘PU’ is used to indicate the inter-coded block unit and sub-PU to indicate the unit that derives the motion information from neighbouring blocks.

Any combination of the following techniques may be applied.

FIG. 8 is a conceptual diagram illustrating an example current prediction unit (PU) 250 and neighboring sub-PUs 252A-252I. Current block 250 includes sub-PUs 254A-254P. When a PU includes multiple sub-PUs, the size of each of the sub-PUs is usually equal to or bigger than that neighboring block size. In the example of FIG. 8, sub-PUs 252A-252I represent neighboring blocks that are outside of current PU 250, and sub-PUS 254A-254P represent the sub-PUs in current PU 250. In this example, the sizes of sub-PUs 254A-254P neighboring sub-PUs 252A-252I are the same. For example, the size may be equal to 4×4.

By contrast, FIG. 9 is a conceptual diagram illustrating another example current PU 260 including sub-PUs 262A-262D that are larger than neighboring blocks 264A-2641. In other examples, sub-PUs may have non-square shapes, such as rectangles or triangles.

In some examples, the sizes of sub-PUs may be signalled in a slice header of a slice including blocks partitioned into the sub-PUs.

Alternatively, the process in bullet #12 of the discussion above related to ATMPV can be extended. Consider the case in FIG. 8, assume the raster scan order (254A, 254B, 254C, 254D, 254E, and so on) is applied to sub-PUs 254A-254P for their motion prediction derivation in the following description. However, other scan orders may be applied also and it should be noted that the techniques of this disclosure are not limited to raster scan order only.

Here, neighboring blocks may be classified into two different types: spatial and temporal. A spatial neighboring block is an already coded block or an already scanned sub-PU that is in the current picture or slice and neighboring to the current sub-PU, such as spatial neighboring sub-PUs 252A-252I of FIG. 8. A temporal neighboring block (not shown in FIG. 8) is a block in the previously coded picture and neighboring to the co-located block of the current sub-PU. In one example, all the reference pictures associated with current PU are used to obtain the temporal neighboring block. In another example, a sub-set of reference pictures are used for STMVP derivation, e.g., only the first entry of each reference picture list is used.

Following this definition, for sub-PU 254A, all neighboring blocks 252A-252P and their collocated blocks in previous coded pictures are spatial and temporal neighboring blocks that are treated as available. According to raster scan order, blocks 254B-254P are not spatially available. Though, all sub-PUs (from 254A to 254P) are temporally available neighboring blocks for sub-PU (A), because their motion information can be found in their collocated blocks in previous coded pictures. Take sub-PU 254G as another example, its spatial neighboring blocks that are available include those from 252A-252I and also from 254A-254F. In some examples, certain restrictions may be applied to the spatial neighbouring blocks, e.g., the spatial neighbouring blocks (i.e., sub-PUs 252A-252I) may be restricted to be in the same LCU/slice/tile as current block 250 in order to be considered “available.”

A subset of all available neighboring blocks may be selected to derive motion information or motion field for each sub-PU. The subset used for derivation of each PU may be pre-defined, alternatively, it may be signalled as high level syntax in slice header/PPS/SPS. To optimize the coding performance, the subset may be different for each sub-PU. In practice, a fixed pattern of location for the subset is preferred for simplicity. For example, each sub-PU may use its immediate above spatial neighbor, its immediate left spatial neighbor and its immediate bottom-right temporal neighbor as the subset. As shown in FIG. 8, when considering sub-PU 254J, the block above (sub-PU 254F) and the block left (sub-PU 254I) are spatially available neighboring blocks and the bottom-right block (sub-PU 254O) is a temporally available neighboring block. With such a subset, sub-PUs 254A-254P in current PU 250 may be processed sequentially, due to processing dependency.

To allow paralleling processing of each of sub-PUs 254A-254P in current PU 250, a different subset of neighboring sub-PUs 252A-252I may be defined and used. In one example, a subset only contains spatial neighbor blocks that do not belong to current PU 250, e.g., neighboring sub-PUs 252A-252I. In this case, parallel processing would be possible. In another example, for a given one of sub-PUs 254A-254P, if its spatial neighboring block is within current PU 250, the collocated block of that spatial neighboring block may be put in the subset and used to derive the motion information of the current sub-PU. For example, when considering sub-PU 254J, the temporal collocated blocks of the above block (sub-PU 254F) and the left block (sub-PU 254I) and bottom-right block (sub-PU 254O) are selected as the subset to derive the motion of the sub-PU (sub-PU 254J). In this case, the subset for sub-PU 254J contains three temporal neighboring blocks. In another example, partially-parallel processing may be enabled wherein one PU is split into several regions and each region (covering several sub-PUs) could be processed independently.

Sometimes the neighboring blocks are intra coded, and it may be desirable to have a rule to determine replacement motion information for those blocks for better motion prediction and coding efficiency. For example, considering sub-PU 254A, there might be cases where sub-PUs 252B, 252C, and 252F are intra-coded, and sub-PUs 252A, 252D, 252E, 252G, 252H, and 252I are inter-coded.

For spatial neighbors, a pre-defined order may be used to populate the motion information of intra-coded blocks with that of the first found inter coded block. For example, the searching order of the above neighbors can be set as starting from the immediate above neighbor rightward until the rightmost neighbor, meaning the order of sub-PUs 252B, 252C, 252D, and 252E. The search order of the left neighbors can be set as starting from the immediate left neighbor downward until the bottommost neighbor, meaning the orders of sub-PUs 252F, 252G, 252H, and 252I. If no inter-coded block is found through the search process, then above or left spatial neighbor is considered unavailable.

For temporal neighbors, the same rule as specified in the TMVP derivation can be used. However, it should be noted that other rules can also be used, e.g., rules based on motion direction, temporal distance (search in different reference pictures) and spatial locations, etc.

For each neighboring block, motion vector scaling is applied to its motion vector based on each reference picture list in order to map all the neighboring blocks' motion vectors to a same reference picture in each list. There are two steps: first, determine a source motion vector to be used for scaling. Second, determine a target reference picture where the source motion vector is projected to. For the first step, several methods can be used.

-   -   (a) for each reference list, motion vector scaling is         independent from motion vector in another reference list; For a         given block's motion information, if there is no motion vector         in a reference list (e.g., uni-prediction mode instead of         bi-prediction mode), no motion vector scaling is performed for         that list.     -   (b) motion vector scaling is not independent from motion vector         in another reference list; for a given block's motion         information, if no motion vector is unavailable in a reference         list, it can be scaled from the one in another reference list.     -   (c) both motion vectors are scaled from one pre-defined         reference list (as in TMVP mentioned above).

As one example, method (a) is used for scaling motion vectors of spatial neighboring blocks, and method (c) is used for scaling motion vectors of temporal neighboring blocks.

As for the second step, the target reference picture can be selected according to a certain rule based on the motion information (e.g., reference pictures) of available spatial neighboring blocks. One example of such a rule is the majority rule, i.e., selecting the reference picture shared by majority of the blocks. In this case there is no signaling needed for the target reference picture from the encoder to decoder because the same information can also be inferred at decoder side using the same rule. Alternatively, such reference picture may also be specified explicitly in slice header, or signalled in some other methods to decoder. The target reference picture is determined as the first reference picture (refidx=0) of each reference list.

After retrieving motion information from neighboring blocks as illustrated in the previous section and motion scaling process (if needed), the motion information of the current sub-PU is derived. Assume there are N available neighboring blocks with motion information for one given sub-PU. First, the prediction direction (InterDir) has to be determined. The simplest method is as follows:

-   -   a. InterDir is initialized as zero, then looping through the         motion information of N available neighboring blocks;     -   b. InterDir=(InterDir bitwiseOR 1), if there is at least one         motion vector in List 0;     -   c. InterDir=(InterDir bitwiseOR 2), if there is at least one         motion vector in List 1.

Here “bitwiseOR” represent the bitwise OR operation. The value of InterDir is defined as: 0 (no inter prediction), 1 (inter prediction based on List 0), 2 (inter prediction based on List 1), and 3 (inter prediction based on both List 0 and List 1).

Alternatively, similar to the determination on target reference picture for motion vector scaling described above, the majority rule may be used to determine the value of InterDir for the given sub-PU based on all available neighboring blocks' motion information.

After InterDir is determined, motion vectors can be derived. For each reference list based on the derived InterDir, there may be M motion vectors (M<=N) available through motion vector scaling to a target reference picture as described above. The motion vector for the reference list can be derived as: (MV _(x) ,MV _(y))=((Σ_(i=0) ^(M) w _(i) *MV _(xi) +O _(i))/Σ_(i=0) ^(M) w _(i),(Σ_(j=0) ^(M) w _(j) *MV _(yj) +O _(j))/Σ_(j=0) ^(M) w _(j))

where w_(i) and w_(j) are the weighting factors for the horizontal and the vertical motion component respectively, and O_(i) and O_(j) are the offset values that are dependent on the weighting factors.

The weighting factors may be determined based on various factors. In one example, the same rule may be applied to all sub-PUs within one PU. The rule may be defined as follows:

For example, the weighting factor can be determined based on the location distance of the current sub-PU and a corresponding neighboring block.

In another example, the weighting factor can also be determined based on the POC distance between the target reference picture and the reference picture associated with a corresponding neighboring block's motion vector before scaling.

In yet another example, the weighting factor may be determined based on motion vector difference or consistency.

For simplicity, all the weighting factors may also be set to 1.

Alternatively, different rules may be applied to sub-PUs within one PU. For example, the above rule may be applied, in addition, for sub-PUs located at the first row/first column, the weighting factors for motion vectors derived from temporal neighboring blocks are set to 0 while for the remaining blocks, the weighting factors for motion vectors derived from spatial neighboring blocks are set to 0.

It should be noted that in practice, the equations above may be implemented as it is, or simplified for easy implementation. For example, to avoid division or floating point operation, fixed point operation may be used to approximate the equation above. One instance is that to avoid divide by 3, one may instead choose to multiply with 43/128 to replace division operation with multiplication and bit-shift. Those variations in implementation should be considered covered under the same spirit of the techniques of this disclosure.

Alternatively, non-linear operation may be also applied to derive the motion vectors, such as median filter.

In some examples, even when the motion vector predictors of each sub-PU are available, the STMVP mode may be reset to be unavailable for one PU.

For example, once a motion vector predictor of each sub-PU is derived for a given PU, some availability checks are performed to determine if STMVP mode should be made available for the given PU. Such an operation is used to eliminate the cases where it is very unlikely for STMVP mode to be finally chosen for a given PU. When STMVP mode is not available, mode signaling does not include STMVP. In case that STMVP mode is implemented by inserting SMTVP in merge list, the merge list doesn't include this STMVP candidate when STMVP mode is determined to be not available. As a result, signaling overhead may be reduced.

Consider one PU partitioned into M sub-PUs. In one example, if N1 (N1<=M) sub-PUs among the M sub-PUs have the same motion vector predictor (i.e., same motion vectors and same reference picture indices), STMVP is only made available when N1 is smaller than a threshold or the predictor is different from other motion vector predictors (with smaller merge index) in the merge list. In another example, if N2 (N2<=M) sub-PUs under STMVP mode share the same motion vector predictors as corresponding sub-PUs under ATMVP, STMVP is only made available when N2 is smaller than another threshold.

In one example of this disclosure, both thresholds for N1 and N2 are set equal to M.

In some examples, if STMVP is available, it is inserted into merge list. The process in bullet #1 above can be extended and STMVP can be inserted either before or after ATMVP. In one example, STMVP is inserted right after ATMVP in the merge list.

When a syntax element is coded with context adaptive binary arithmetic coding (CABAC), a context model is applied to represent conditional probability. In HEVC, a CABAC coder determines different context models for different syntax elements. The CABAC coder can choose one context model from several candidate context models for a syntax element in some examples, based on the coding context, such as the bin number or information of decoded neighboring blocks. For example, three candidate context models named skip_flag_C[0], skip_flag_C[1] and skip_flag_C[2] can be used to code the syntax element cu_skip_flag which indicates whether one CU is coded with skip mode or not.

To choose the appropriate context from the three candidate, the CABAC coder may calculate context index x as: x=(cu_skip_flag[xNbL][yNbL]&& availableL)+(cu_skip_flag[xNbA][yNbA]&& availableA) wherein the luma location (x0, y0) specifies the top-left luma sample of the current luma block relative to the top-left sample of the current picture; the location xNbL, yNbL) is set equal to (x0−1, y0) and the variable availableL, specifying the availability of the block located directly to the left of the current block, i.e., block L in FIG. 10; the location (xNbA, yNbA) is set equal to (x0, y0−1) and the variable availableA specifying the availability of the coding block located directly above the current block, i.e., block A in FIG. 10 and cu_skip_flag[xNbL][yNbL] and cu_skip_flag[xNbA][yNbA] represent the cu_skip_flag of the left block L and above block A in FIG. 10 respectively. Neighboring blocks used to derive the context information of cu_skip_flag is illustrated in FIG. 10.

As described above, there are many priority-based candidate lists. Each candidate is inserted into the candidate list according to a predefined priority. For example, in HEVC, Merge candidate list, AMVP candidate list, intra most probable mode (MPM) list are constructed by inserting candidates based on a predefined order (or according to a predefined priority). As shown in FIG. 11, the merge candidate list is constructed by inserting the spatial merging candidate by a predefined order (A1→B1→B0→A0→B2). Such a fixed order may not be able to capture local characteristics. If flexible order could be applied by putting a candidate with higher chance to be selected before other candidates, higher coding performance may be expected.

In techniques of this disclosure, geometry information may be used between the current block and the neighboring blocks to determine the priority or insertion order for the construction of candidate lists such as merging candidate list, AMVP list and intra MPM list. Moreover, this geometry information can be used for the determination of the context for CABAC coding.

The following itemized methods may be applied individually. Alternatively, any combination of them may be applied.

(1) In one example, the distance between a representative point of current block and a representative point of the neighboring block which the candidate belongs to is used as the geometry information to determine the priority or insertion order for the construction of the candidate lists. The term “block” (e.g., Block0-Block4 and Current Block in FIG. 12) used here can be a coding unit/block, prediction unit/block, sub-PU, transform unit/block or any other coding structure. Moreover, the unit block is the basic unit to store the coding information such as motion information (motion vectors, reference picture index, inter prediction directions and so on), intra prediction modes, transform information and so on. For example, the size of this unit block may be 4×4. As shown in FIG. 12, the candidates A0, A1, B0, B1 and B2 are derived from the unit blocks covered by the neighboring blocks Block0, Block1, Block2, Block3 and Block4, respectively.

-   -   a. In one example, the shorter distance between that candidate's         representative point and current representative point has higher         priority or vice versa.     -   b. In one example, the distance can be LN-norm distance (N can         be 1, 2 or any other positive integer).

(2) In item 1, the representative point of current block can be any point within current block. In one example, as shown in FIG. 12, the representative point of current block is the center point of current block. FIG. 12 illustrates an example of geometric information of spatial merging candidates.

(3) In item 1, the representative point of the neighboring block can be any point within the neighboring block. In one example, as shown in FIG. 12, the representative point of a neighboring block is the center point of the neighboring block.

(4) Alternatively, as shown in FIG. 13, the representative point of a neighboring block is the center point of the sub-PU covered by the neighboring block. For example, if neighbor blocks are coded as sub-PU modes such as FRUC, Affine, ATMVP, the center point of that sub-PU is used as the representative point for that block. FIG. 13 illustrates an example of geometric information of spatial merging candidates. As shown in FIG. 13, since Block 1 is coded as sub-PU mode, the representative point of the sub-PU which candidate A1 belongs to is used to derive the geometric priority.

(5) Additionally or alternatively, the representative point can be any point within current block or the neighboring block determined adaptively according to coding information such as MV, reference picture information, intra modes, transform coefficients, residual information, and so on.

-   -   a. In one example, for constructing the intra mode candidate         list, the center point may be replaced by the top-left point         within a block.

(6) The above mentioned methods can be applied to determine the priority to construct the candidate list, individually or in any combination of the other priorities.

-   -   a. In one example, when two or more candidates have the same         geometry priority (e.g., same distance between representative         points), a predefined priority can be used to distinguish them.         For example, an insertion order (A1→B1→B0→A0→B2) can be used.     -   b. Alternatively, the other priority can be the coding         information such as inter prediction direction (e.g., L0, L1 or         bi), the MVs, the intra prediction modes, the POC of the         reference pictures and so on.

(7) The above mentioned methods may be applied to certain blocks.

-   -   a. In one example, the above method is applied to blocks with         different width and height.     -   b. Alternatively, the above method is applied to blocks with the         ratio of width and height larger than K or smaller than 1/K         wherein K is a positive integer value and larger than 1.     -   c. Alternatively, the above method is applied to blocks with         certain sizes.

(8) The above mentioned methods may be partially applied to some of the candidates.

-   -   a. In one example, the geometry information is only used to         determine the order of spatial merging/intra mode candidates,         i.e., candidates derived from spatial neighboring blocks.     -   b. In one example, the geometry information is only used to         determine the order of the paired candidates (A1, B1).     -   c. In one example, the geometry information is only used to         determine the order of the paired candidates (A0, B0).

(9) Whether to use the geometry-based priority may be signaled in the bitstream.

-   -   a. In one example, for each coding unit/prediction         unit/transform unit, a flag may be signaled to indicate whether         the geometry-based priority or the predefined priority list         should be used.

(10) When the order of merge candidate list is modified according to the geometry-based priority, the first candidate after re-ordering is used in the first stage for ATMVP and/or STMVP processes.

-   -   a. Alternatively, the first candidate derived from the         predefined order is used in the first stage for ATMVP and/or         STMVP processes.

(11) The geometry information may be used for the determination of contexts for CABAC coding when information from neighboring blocks are utilized.

-   -   a. In one example, in addition to the left (such as L depicted         in FIG. 10) and the above block (such as A depicted in FIG. 10),         more neighboring blocks such as A0, A1, B0, B1 and B2 (same as A         in FIG. 10) as depicted in FIG. 11 are used to derive the         context of cu_skip_flag of the current block.     -   b. When multiple neighboring blocks (denote the total number         by M) are utilized for context modeling, only the information         from the first N blocks based on the geometry priority is         considered. Here, N is smaller than M. In one example, N is set         to 2 and M is set to 5.     -   c. Alternatively, furthermore, context modeling of other syntax         elements, which include but not limited to         cu_transquant_bypass_flag, cu_skip_flag, cbf, pred_mode_flag,         rqt_root_cbf, merge_idx, merge_flag, cbf_luma, cbf_cb, cbf_cr         may also use the above method.     -   d. In one example, the above method may be applied to certain         blocks, e.g., the width and height of a block is different, or         the ratio of width and height is larger than a threshold or         applied to blocks with certain sizes.     -   e. In one example, the CABAC context is determined by the coding         information of the neighboring blocks scaled by their associated         weighting factors. The associated weighting factors is derived         with the geometry information. For example, when selecting the         context for coding skip flag, x may be derived by the following         equation.         x=WL*(cu_skip_flag[xNbL][yNbL]&&         availableL)+WA(cu_skip_flag[xNbA][yNbA]&& availableA)         The WL and WA are the associated weighting factor for left and         above neighboring blocks, respectively. WL and WA can be derived         according to their geometry information. In one example, when         the distance between the representative point of neighbor block         and current block is larger than a predefined threshold, the         associated weighting factor is set to a value M; otherwise (when         the distance is less than or equal to a predefined threshold),         the associated weighting factor is set to a value N.     -   f. In item 11.e, the neighboring blocks are not limited to left         and above blocks used in HEVC. The neighboring blocks can be any         one of the previously coded blocks.     -   g. In item 11.e, more values can be assigned for weighting         factors by introducing more thresholds.

(12) In another example, the geometry information of the neighboring blocks may be used to determine the priority or insertion order for the construction of candidate lists such as merging candidate list, AMVP list and intra MPM list. Moreover, this geometry information can be used for the determination of the context for CABAC coding.

(13) In one example, the area of the neighboring block which the candidate belongs to is used as a geometry information to determine the priority or insertion order for the construction of the candidate lists. The term “block” used here can be coding unit/block, prediction unit/block, sub-PU, transform unit/block or any other coding structures. The block with smaller area has higher priority or vice versa.

(14) The area-based geometry information can be applied as the same as the aforementioned methods as described in items 6 to 11.

FIG. 14 is a block diagram illustrating an example video encoding and decoding system 10 that may be configured to perform the techniques of this disclosure for motion vector prediction. As shown in FIG. 14, system 10 includes a source device 12 that provides encoded video data to be decoded at a later time by a destination device 14. In particular, source device 12 provides the video data to destination device 14 via a computer-readable medium 16. Source device 12 and destination device 14 may comprise any of a wide range of devices, including desktop computers, notebook (i.e., laptop) computers, tablet computers, set-top boxes, telephone handsets such as so-called “smart” phones, so-called “smart” pads, televisions, cameras, display devices, digital media players, video gaming consoles, video streaming device, or the like. In some cases, source device 12 and destination device 14 may be equipped for wireless communication.

Destination device 14 may receive the encoded video data to be decoded via computer-readable medium 16. Computer-readable medium 16 may comprise any type of medium or device capable of moving the encoded video data from source device 12 to destination device 14. In one example, computer-readable medium 16 may comprise a communication medium to enable source device 12 to transmit encoded video data directly to destination device 14 in real-time. The encoded video data may be modulated according to a communication standard, such as a wireless communication protocol, and transmitted to destination device 14. The communication medium may comprise any wireless or wired communication medium, such as a radio frequency (RF) spectrum or one or more physical transmission lines. The communication medium may form part of a packet-based network, such as a local area network, a wide-area network, or a global network such as the Internet. The communication medium may include routers, switches, base stations, or any other equipment that may be useful to facilitate communication from source device 12 to destination device 14.

In some examples, encoded data may be output from output interface 22 to a storage device. Similarly, encoded data may be accessed from the storage device by input interface. The storage device may include any of a variety of distributed or locally accessed data storage media such as a hard drive, Blu-ray discs, DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or any other suitable digital storage media for storing encoded video data. In a further example, the storage device may correspond to a file server or another intermediate storage device that may store the encoded video generated by source device 12. Destination device 14 may access stored video data from the storage device via streaming or download. The file server may be any type of server capable of storing encoded video data and transmitting that encoded video data to the destination device 14. Example file servers include a web server (e.g., for a website), an FTP server, network attached storage (NAS) devices, or a local disk drive. Destination device 14 may access the encoded video data through any standard data connection, including an Internet connection. This may include a wireless channel (e.g., a Wi-Fi connection), a wired connection (e.g., DSL, cable modem, etc.), or a combination of both that is suitable for accessing encoded video data stored on a file server. The transmission of encoded video data from the storage device may be a streaming transmission, a download transmission, or a combination thereof.

The techniques of this disclosure are not necessarily limited to wireless applications or settings. The techniques may be applied to video coding in support of any of a variety of multimedia applications, such as over-the-air television broadcasts, cable television transmissions, satellite television transmissions, Internet streaming video transmissions, such as dynamic adaptive streaming over HTTP (DASH), digital video that is encoded onto a data storage medium, decoding of digital video stored on a data storage medium, or other applications. In some examples, system 10 may be configured to support one-way or two-way video transmission to support applications such as video streaming, video playback, video broadcasting, and/or video telephony.

In the example of FIG. 14, source device 12 includes video source 18, video encoder 20, and output interface 22. Destination device 14 includes input interface 28, video decoder 30, and display device 32. In accordance with this disclosure, video encoder 20 of source device 12 and video decoder 30 of destination device 14 may be configured to apply the candidate list construction techniques of this disclosure for motion vector prediction. In other examples, a source device and a destination device may include other components or arrangements. For example, source device 12 may receive video data from an external video source 18, such as an external camera. Likewise, destination device 14 may interface with an external display device, rather than including an integrated display device.

The illustrated system 10 of FIG. 14 is merely one example. The techniques of this disclosure for motion vector prediction may be performed by any digital video encoding and/or decoding device. Although generally the techniques of this disclosure are performed by a video encoding device, the techniques may also be performed by a video encoder/decoder, typically referred to as a “CODEC.” Moreover, the techniques of this disclosure may also be performed by a video preprocessor. Source device 12 and destination device 14 are merely examples of such coding devices in which source device 12 generates coded video data for transmission to destination device 14. In some examples, devices 12, 14 may operate in a substantially symmetrical manner such that each of devices 12, 14 include video encoding and decoding components. Hence, system 10 may support one-way or two-way video transmission between video devices 12, 14, e.g., for video streaming, video playback, video broadcasting, or video telephony.

Video source 18 of source device 12 may include a video capture device, such as a video camera, a video archive containing previously captured video, and/or a video feed interface to receive video from a video content provider. As a further alternative, video source 18 may generate computer graphics-based data as the source video, or a combination of live video, archived video, and computer-generated video. In some cases, if video source 18 is a video camera, source device 12 and destination device 14 may form so-called camera phones or video phones. As mentioned above, however, the techniques described in this disclosure may be applicable to video coding in general, and may be applied to wireless and/or wired applications. In each case, the captured, pre-captured, or computer-generated video may be encoded by video encoder 20. The encoded video information may then be output by output interface 22 onto a computer-readable medium 16.

Computer-readable medium 16 may include transient media, such as a wireless broadcast or wired network transmission, or storage media (that is, non-transitory storage media), such as a hard disk, flash drive, compact disc, digital video disc, Blu-ray disc, or other computer-readable media. In some examples, a network server (not shown) may receive encoded video data from source device 12 and provide the encoded video data to destination device 14, e.g., via network transmission. Similarly, a computing device of a medium production facility, such as a disc stamping facility, may receive encoded video data from source device 12 and produce a disc containing the encoded video data. Therefore, computer-readable medium 16 may be understood to include one or more computer-readable media of various forms, in various examples.

Input interface 28 of destination device 14 receives information from computer-readable medium 16. The information of computer-readable medium 16 may include syntax information defined by video encoder 20, which is also used by video decoder 30, that includes syntax elements that describe characteristics and/or processing of blocks and other coded units, e.g., GOPs. Display device 32 displays the decoded video data to a user, and may comprise any of a variety of display devices such as a cathode ray tube (CRT), a liquid crystal display (LCD), a plasma display, an organic light emitting diode (OLED) display, or another type of display device.

Video encoder 20 and video decoder 30 may operate according to a video coding standard, such as the High Efficiency Video Coding (HEVC) standard, extensions to the HEVC standard, or subsequent standards, such as ITU-T H.266. Alternatively, video encoder 20 and video decoder 30 may operate according to other proprietary or industry standards, such as the ITU-T H.264 standard, alternatively referred to as MPEG-4, Part 10, Advanced Video Coding (AVC), or extensions of such standards. The techniques of this disclosure, however, are not limited to any particular coding standard. Other examples of video coding standards include MPEG-2 and ITU-T H.263. Although not shown in FIG. 14, in some aspects, video encoder 20 and video decoder 30 may each be integrated with an audio encoder and decoder, and may include appropriate MUX-DEMUX units, or other hardware and software, to handle encoding of both audio and video in a common data stream or separate data streams. If applicable, MUX-DEMUX units may conform to the ITU H.223 multiplexer protocol, or other protocols such as the user datagram protocol (UDP).

Video encoder 20 and video decoder 30 each may be implemented as any of a variety of suitable encoder circuitry, such as one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), discrete logic, software, hardware, firmware or any combinations thereof. When the techniques are implemented partially in software, a device may store instructions for the software in a suitable, non-transitory computer-readable medium and execute the instructions in hardware using one or more processors to perform the techniques of this disclosure. Each of video encoder 20 and video decoder 30 may be included in one or more encoders or decoders, either of which may be integrated as part of a combined encoder/decoder (CODEC) in a respective device.

Video coding standards include ITU-T H.261, ISO/IEC MPEG-1 Visual, ITU-T H.262 or ISO/IEC MPEG-2 Visual, ITU-T H.263, ISO/IEC MPEG-4 Visual and ITU-T H.264 (also known as ISO/IEC MPEG-4 AVC), including its Scalable Video Coding (SVC) and Multiview Video Coding (MVC) extensions. One joint draft of MVC is described in “Advanced video coding for generic audiovisual services,” ITU-T Recommendation H.264, March, 2010.

In addition, there is a newly developed video coding standard, namely High Efficiency Video Coding (HEVC), developed by the Joint Collaboration Team on Video Coding (JCT-VC) of ITU-T Video Coding Experts Group (VCEG) and ISO/IEC Motion Picture Experts Group (MPEG). A recent draft of HEVC is available fromphenix.int-evry.fr/jct/doc_end_user/documents/12_Geneva/wg11/JCTVC-L1003-v34.zip. The HEVC standard is also presented jointly in Recommendation ITU-T H.265 and International Standard ISO/IEC 23008-2, both entitled “High efficiency video coding,” and both published October, 2014.

The JCT-VC developed the HEVC standard. The HEVC standardization efforts are based on an evolving model of a video coding device referred to as the HEVC Test Model (HM). The HM presumes several additional capabilities of video coding devices relative to existing devices according to, e.g., ITU-T H.264/AVC. For example, whereas H.264 provides nine intra-prediction encoding modes, the HEVC HM may provide as many as thirty-three intra-prediction encoding modes.

In general, the working model of the HM describes that a video frame or picture may be divided into a sequence of treeblocks or largest coding units (LCU) that include both luma and chroma samples. Syntax data within a bitstream may define a size for the LCU, which is a largest coding unit in terms of the number of pixels. A slice includes a number of consecutive treeblocks in coding order. A video frame or picture may be partitioned into one or more slices. Each treeblock may be split into coding units (CUs) according to a quadtree. In general, a quadtree data structure includes one node per CU, with a root node corresponding to the treeblock. If a CU is split into four sub-CUs, the node corresponding to the CU includes four leaf nodes, each of which corresponds to one of the sub-CUs.

Each node of the quadtree data structure may provide syntax data for the corresponding CU. For example, a node in the quadtree may include a split flag, indicating whether the CU corresponding to the node is split into sub-CUs. Syntax elements for a CU may be defined recursively, and may depend on whether the CU is split into sub-CUs. If a CU is not split further, it is referred as a leaf-CU. In this disclosure, four sub-CUs of a leaf-CU will also be referred to as leaf-CUs even if there is no explicit splitting of the original leaf-CU. For example, if a CU at 16×16 size is not split further, the four 8×8 sub-CUs will also be referred to as leaf-CUs although the 16×16 CU was never split.

A CU has a similar purpose as a macroblock of the H.264 standard, except that a CU does not have a size distinction. For example, a treeblock may be split into four child nodes (also referred to as sub-CUs), and each child node may in turn be a parent node and be split into another four child nodes. A final, unsplit child node, referred to as a leaf node of the quadtree, comprises a coding node, also referred to as a leaf-CU. Syntax data associated with a coded bitstream may define a maximum number of times a treeblock may be split, referred to as a maximum CU depth, and may also define a minimum size of the coding nodes. Accordingly, a bitstream may also define a smallest coding unit (SCU). This disclosure uses the term “block” to refer to any of a CU, PU, or TU, in the context of HEVC, or similar data structures in the context of other standards (e.g., macroblocks and sub-blocks thereof in H.264/AVC).

A CU includes a coding node and prediction units (PUs) and transform units (TUs) associated with the coding node. A size of the CU corresponds to a size of the coding node and must be square in shape. The size of the CU may range from 8×8 pixels up to the size of the treeblock with a maximum of 64×64 pixels or greater. Each CU may contain one or more PUs and one or more TUs. Syntax data associated with a CU may describe, for example, partitioning of the CU into one or more PUs. Partitioning modes may differ between whether the CU is skip or direct mode encoded, intra-prediction mode encoded, or inter-prediction mode encoded. PUs may be partitioned to be non-square in shape. Syntax data associated with a CU may also describe, for example, partitioning of the CU into one or more TUs according to a quadtree. A TU can be square or non-square (e.g., rectangular) in shape.

The HEVC standard allows for transformations according to TUs, which may be different for different CUs. The TUs are typically sized based on the size of PUs within a given CU defined for a partitioned LCU, although this may not always be the case. The TUs are typically the same size or smaller than the PUs. In some examples, residual samples corresponding to a CU may be subdivided into smaller units using a quadtree structure known as “residual quad tree” (RQT). The leaf nodes of the RQT may be referred to as transform units (TUs). Pixel difference values associated with the TUs may be transformed to produce transform coefficients, which may be quantized.

A leaf-CU may include one or more prediction units (PUs). In general, a PU represents a spatial area corresponding to all or a portion of the corresponding CU, and may include data for retrieving a reference sample for the PU. Moreover, a PU includes data related to prediction. For example, when the PU is intra-mode encoded, data for the PU may be included in a residual quadtree (RQT), which may include data describing an intra-prediction mode for a TU corresponding to the PU. As another example, when the PU is inter-mode encoded, the PU may include data defining one or more motion vectors for the PU. The data defining the motion vector for a PU may describe, for example, a horizontal component of the motion vector, a vertical component of the motion vector, a resolution for the motion vector (e.g., one-quarter pixel precision or one-eighth pixel precision), a reference picture to which the motion vector points, and/or a reference picture list (e.g., List 0, List 1, or List C) for the motion vector.

Geometry information between a current block and neighboring blocks may determine priority or insertion order for the construction of the reference picture list (e.g., List 0, List 1, or List C) for the motion vector. Geometry information may include the distance between a representative point of the current block (e.g., a central point) and a representative point of the neighboring block which the candidate belongs to. A higher priority may be indicated for neighboring blocks with a shorter distance between the representative point and the representative point of the current block. The representative point may be the any point (e.g., a central point) within the block.

A leaf-CU having one or more PUs may also include one or more transform units (TUs). The transform units may be specified using an RQT (also referred to as a TU quadtree structure), as discussed above. For example, a split flag may indicate whether a leaf-CU is split into four transform units. Then, each transform unit may be split further into further sub-TUs. When a TU is not split further, it may be referred to as a leaf-TU. Generally, for intra coding, all the leaf-TUs belonging to a leaf-CU share the same intra prediction mode. That is, the same intra-prediction mode is generally applied to calculate predicted values for all TUs of a leaf-CU. For intra coding, a video encoder may calculate a residual value for each leaf-TU using the intra prediction mode, as a difference between the portion of the CU corresponding to the TU and the original block. A TU is not necessarily limited to the size of a PU. Thus, TUs may be larger or smaller than a PU. For intra coding, a PU may be collocated with a corresponding leaf-TU for the same CU. In some examples, the maximum size of a leaf-TU may correspond to the size of the corresponding leaf-CU.

Moreover, TUs of leaf-CUs may also be associated with respective quadtree data structures, referred to as residual quadtrees (RQTs). That is, a leaf-CU may include a quadtree indicating how the leaf-CU is partitioned into TUs. The root node of a TU quadtree generally corresponds to a leaf-CU, while the root node of a CU quadtree generally corresponds to a treeblock (or LCU). TUs of the RQT that are not split are referred to as leaf-TUs. In general, this disclosure uses the terms CU and TU to refer to leaf-CU and leaf-TU, respectively, unless noted otherwise.

A video sequence typically includes a series of video frames or pictures. A group of pictures (GOP) generally comprises a series of one or more of the video pictures. A GOP may include syntax data in a header of the GOP, a header of one or more of the pictures, or elsewhere, that describes a number of pictures included in the GOP. Each slice of a picture may include slice syntax data that describes an encoding mode for the respective slice. Video encoder 20 typically operates on video blocks within individual video slices in order to encode the video data. A video block may correspond to a coding node within a CU. The video blocks may have fixed or varying sizes, and may differ in size according to a specified coding standard.

As an example, the HM supports prediction in various PU sizes. Assuming that the size of a particular CU is 2N×2N, the HM supports intra-prediction in PU sizes of 2N×2N or N×N, and inter-prediction in symmetric PU sizes of 2N×2N, 2N×N, N×2N, or N×N. The HM also supports asymmetric partitioning for inter-prediction in PU sizes of 2N×nU, 2N×nD, nL×2N, and nR×2N. In asymmetric partitioning, one direction of a CU is not partitioned, while the other direction is partitioned into 25% and 75%. The portion of the CU corresponding to the 25% partition is indicated by an “n” followed by an indication of “Up”, “Down,” “Left,” or “Right.” Thus, for example, “2N×nU” refers to a 2N×2N CU that is partitioned horizontally with a 2N×0.5N PU on top and a 2N×1.5N PU on bottom.

In general, intra-prediction involves predicting a block using neighboring, previously coded pixels to the block (within the same picture). Various intra-prediction modes, such as horizontal, vertical, and various diagonal modes, as well as DC and planar modes, may be used. Furthermore, certain modes may be considered “most probable,” based on intra-prediction modes used to intra-predict neighboring blocks. In accordance with the techniques of this disclosure, video encoder 20 and video decoder 30 may construct a most probable mode (MPM) list including neighboring blocks to the current block as candidates, such that candidates within the MPM list are ordered, e.g., according to geometry information for the current block and the neighboring blocks, as discussed above.

In this disclosure, “N×N” and “N by N” may be used interchangeably to refer to the pixel dimensions of a video block in terms of vertical and horizontal dimensions, e.g., 16×16 pixels or 16 by 16 pixels. In general, a 16×16 block will have 16 pixels in a vertical direction (y=16) and 16 pixels in a horizontal direction (x=16). Likewise, an N×N block generally has N pixels in a vertical direction and N pixels in a horizontal direction, where N represents a nonnegative integer value. The pixels in a block may be arranged in rows and columns. Moreover, blocks need not necessarily have the same number of pixels in the horizontal direction as in the vertical direction. For example, blocks may comprise N×M pixels, where M is not necessarily equal to N.

Following intra-predictive or inter-predictive coding using the PUs of a CU, video encoder 20 may calculate residual data for the TUs of the CU. The PUs may comprise syntax data describing a method or mode of generating predictive pixel data in the spatial domain (also referred to as the pixel domain) and the TUs may comprise coefficients in the transform domain following application of a transform, e.g., a discrete cosine transform (DCT), an integer transform, a wavelet transform, or a conceptually similar transform to residual video data. The residual data may correspond to pixel differences between pixels of the unencoded picture and prediction values corresponding to the PUs. Video encoder 20 may form the TUs including the residual data for the CU, and then transform the TUs to produce transform coefficients for the CU.

Following any transforms to produce transform coefficients, video encoder 20 may perform quantization of the transform coefficients. Quantization generally refers to a process in which transform coefficients are quantized to possibly reduce the amount of data used to represent the coefficients, providing further compression. The quantization process may reduce the bit depth associated with some or all of the coefficients. For example, an n-bit value may be rounded down to an m-bit value during quantization, where n is greater than m.

Following quantization, the video encoder may scan the transform coefficients, producing a one-dimensional vector from the two-dimensional matrix including the quantized transform coefficients. The scan may be designed to place higher energy (and therefore lower frequency) coefficients at the front of the array and to place lower energy (and therefore higher frequency) coefficients at the back of the array. In some examples, video encoder 20 may utilize a predefined scan order to scan the quantized transform coefficients to produce a serialized vector that can be entropy encoded. In other examples, video encoder 20 may perform an adaptive scan. After scanning the quantized transform coefficients to form a one-dimensional vector, video encoder 20 may entropy encode the one-dimensional vector, e.g., according to context-adaptive variable length coding (CAVLC), context-adaptive binary arithmetic coding (CABAC), syntax-based context-adaptive binary arithmetic coding (SBAC), Probability Interval Partitioning Entropy (PIPE) coding or another entropy encoding methodology. Video encoder 20 may also entropy encode syntax elements associated with the encoded video data for use by video decoder 30 in decoding the video data.

To perform CABAC, video encoder 20 may assign a context within a context model to a symbol to be transmitted. The context may relate to, for example, whether neighboring values of the symbol are non-zero or not. To perform CAVLC, video encoder 20 may select a variable length code for a symbol to be transmitted. Codewords in VLC may be constructed such that relatively shorter codes correspond to more probable symbols, while longer codes correspond to less probable symbols. In this way, the use of VLC may achieve a bit savings over, for example, using equal-length codewords for each symbol to be transmitted. The probability determination may be based on a context assigned to the symbol.

FIG. 15 is a block diagram illustrating an example of video encoder 20 that may be configured to perform the techniques of this disclosure for geometry-based priority lists. Video encoder 20 may perform intra- and inter-coding of video blocks within video slices. Intra-coding relies on spatial prediction to reduce or remove spatial redundancy in video within a given video frame or picture. Inter-coding relies on temporal prediction to reduce or remove temporal redundancy in video within adjacent frames or pictures of a video sequence. Intra-mode (I mode) may refer to any of several spatial based coding modes. Inter-modes, such as uni-directional prediction (P mode) or bi-prediction (B mode), may refer to any of several temporal-based coding modes.

As shown in FIG. 15, video encoder 20 receives a current video block within a video frame to be encoded. In the example of FIG. 15, video encoder 20 includes mode select unit 40, reference picture memory 64, summer 50, transform processing unit 52, quantization unit 54, and entropy encoding unit 56. Mode select unit 40, in turn, includes motion compensation unit 44, motion estimation unit 42, intra-prediction unit 46, and partition unit 48. For video block reconstruction, video encoder 20 also includes inverse quantization unit 58, inverse transform unit 60, and summer 62. A deblocking filter (not shown in FIG. 15) may also be included to filter block boundaries to remove blockiness artifacts from reconstructed video. If desired, the deblocking filter would typically filter the output of summer 62. Additional filters (in loop or post loop) may also be used in addition to the deblocking filter. Such filters are not shown for brevity, but if desired, may filter the output of summer 50 (as an in-loop filter).

During the encoding process, video encoder 20 receives a video frame or slice to be coded. The frame or slice may be divided into multiple video blocks. Motion estimation unit 42 and motion compensation unit 44 perform inter-predictive coding of the received video block relative to one or more blocks in one or more reference frames to provide temporal prediction. Intra-prediction unit 46 may alternatively perform intra-predictive coding of the received video block relative to one or more neighboring blocks in the same frame or slice as the block to be coded to provide spatial prediction. Video encoder 20 may perform multiple coding passes, e.g., to select an appropriate coding mode for each block of video data.

Moreover, partition unit 48 may partition blocks of video data into sub-blocks, based on evaluation of previous partitioning schemes in previous coding passes. For example, partition unit 48 may initially partition a frame or slice into LCUs, and partition each of the LCUs into sub-CUs based on rate-distortion analysis (e.g., rate-distortion optimization). Mode select unit 40 may further produce a quadtree data structure indicative of partitioning of an LCU into sub-CUs. Leaf-node CUs of the quadtree may include one or more PUs and one or more TUs.

Mode select unit 40 may select one of the coding modes, intra or inter, e.g., based on error results, and provides the resulting intra- or inter-coded block to summer 50 to generate residual block data and to summer 62 to reconstruct the encoded block for use as a reference frame. Mode select unit 40 also provides syntax elements, such as motion vectors, intra-mode indicators, partition information, and other such syntax information, to entropy encoding unit 56.

Motion estimation unit 42 and motion compensation unit 44 may be highly integrated, but are illustrated separately for conceptual purposes. Motion estimation, performed by motion estimation unit 42, is the process of generating motion vectors, which estimate motion for video blocks. A motion vector, for example, may indicate the displacement of a PU of a video block within a current video frame or picture relative to a predictive block within a reference frame (or other coded unit) relative to the current block being coded within the current frame (or other coded unit). A predictive block is a block that is found to closely match the block to be coded, in terms of pixel difference, which may be determined by sum of absolute difference (SAD), sum of square difference (SSD), or other difference metrics. In some examples, video encoder 20 may calculate values for sub-integer pixel positions of reference pictures stored in reference picture memory 64. For example, video encoder 20 may interpolate values of one-quarter pixel positions, one-eighth pixel positions, or other fractional pixel positions of the reference picture. Therefore, motion estimation unit 42 may perform a motion search relative to the full pixel positions and fractional pixel positions and output a motion vector with fractional pixel precision.

Motion estimation unit 42 calculates a motion vector for a PU of a video block in an inter-coded slice by comparing the position of the PU to the position of a predictive block of a reference picture. The reference picture may be selected from a first reference picture list (List 0) or a second reference picture list (List 1), each of which identify one or more reference pictures stored in reference picture memory 64. Motion estimation unit 42 sends the calculated motion vector to entropy encoding unit 56 and motion compensation unit 44.

Motion compensation, performed by motion compensation unit 44, may involve fetching or generating the predictive block based on the motion vector determined by motion estimation unit 42. Again, motion estimation unit 42 and motion compensation unit 44 may be functionally integrated, in some examples. Upon receiving the motion vector for the PU of the current video block, motion compensation unit 44 may locate the predictive block to which the motion vector points in one of the reference picture lists. Summer 50 forms a residual video block by subtracting pixel values of the predictive block from the pixel values of the current video block being coded, forming pixel difference values, as discussed below. In general, motion estimation unit 42 performs motion estimation relative to luma components, and motion compensation unit 44 uses motion vectors calculated based on the luma components for both chroma components and luma components. Mode select unit 40 may also generate syntax elements associated with the video blocks and the video slice for use by video decoder 30 in decoding the video blocks of the video slice.

Furthermore, entropy encoding unit 56, or motion compensation unit 44, may apply the techniques of this disclosure when performing motion vector prediction, e.g., using merge mode or AMVP mode. In particular, when constructing a candidate list (e.g., a merge candidate list or an AMVP candidate list) used to predict a motion vector for a current block, entropy encoding unit 56 or motion compensation unit 44 may arrange candidates within the candidate list according to priority values for the candidates, where the priority values may represent geometry information for the candidates, as discussed in this disclosure. The geometry information may be, for example, distances between representative points of a current block and neighboring blocks to the current block (which may include spatial and/or temporal neighboring blocks). As discussed above, the representative points may be center points of the blocks, upper-left points of the blocks, or the like. Entropy encoding unit 56 or motion compensation unit 44 may use any of the various techniques of this disclosure, alone or in any combination, when calculating the priority values.

Video encoder 20 may be configured to perform any of the various techniques of this disclosure discussed above with respect to FIG. 14, and as will be described in more detail below. For example, motion compensation unit 44 may be configured to code motion information for a block of video data using AMVP or merge mode in accordance with the techniques of this disclosure. Additionally or alternatively, intra prediction unit 46 may be configured to code an intra prediction mode in accordance with the techniques of this disclosure. Additionally or alternatively, entropy encoding unit 56 may be configured to determine context information for CABAC coding using the techniques of this disclosure.

For example, assuming that motion compensation unit 44 elects to perform merge mode, motion compensation unit 44 may form a candidate list including a set of merge candidates. Motion compensation unit 44 may add candidates to the candidate list based on a particular, predetermined order. Motion compensation unit 44 may also add additional candidates and perform pruning of the candidate list and prioritize the candidate list, as discussed above. Ultimately, mode select unit 40 may determine which of the candidates is to be used to encode motion information of the current block, and encode a merge index representing the selected candidate.

Intra-prediction unit 46 may intra-predict a current block, as an alternative to the inter-prediction performed by motion estimation unit 42 and motion compensation unit 44, as described above. In particular, intra-prediction unit 46 may determine an intra-prediction mode to use to encode a current block. In some examples, intra-prediction unit 46 may encode a current block using various intra-prediction modes, e.g., during separate encoding passes, and intra-prediction unit 46 (or mode select unit 40, in some examples) may select an appropriate intra-prediction mode to use from the tested modes.

For example, intra-prediction unit 46 may calculate rate-distortion values using a rate-distortion analysis for the various tested intra-prediction modes, and select the intra-prediction mode having the best rate-distortion characteristics among the tested modes. Rate-distortion analysis generally determines an amount of distortion (or error) between an encoded block and an original, unencoded block that was encoded to produce the encoded block, as well as a bitrate (that is, a number of bits) used to produce the encoded block. Intra-prediction unit 46 may calculate ratios from the distortions and rates for the various encoded blocks to determine which intra-prediction mode exhibits the best rate-distortion value for the block.

After selecting an intra-prediction mode for a block, intra-prediction unit 46 may provide information indicative of the selected intra-prediction mode for the block to entropy encoding unit 56. Entropy encoding unit 56 may encode the information indicating the selected intra-prediction mode. As discussed above, intra prediction unit 46 and/or entropy encoding unit 56 may use the techniques of this disclosure to encode the information indicating the selected intra-prediction mode. In particular, entropy encoding unit 56 may determine one or more most probable modes from neighboring blocks to the block based on geometry information, e.g., distances between representative points of the block and the neighboring blocks. Entropy encoding unit 56 may further entropy encode data indicating whether the intra prediction mode used to intra predict the block is one of the most probable modes, or a different mode, and if a different mode, an index into a list of intra prediction modes excluding the most probable modes.

Video encoder 20 may include in the transmitted bitstream configuration data, which may include a plurality of intra-prediction mode index tables and a plurality of modified intra-prediction mode index tables (also referred to as codeword mapping tables), definitions of encoding contexts for various blocks, and indications of a most probable intra-prediction mode, an intra-prediction mode index table, and a modified intra-prediction mode index table to use for each of the contexts.

Video encoder 20 forms a residual video block by subtracting the prediction data from mode select unit 40 from the original video block being coded. Summer 50 represents the component or components that perform this subtraction operation. Transform processing unit 52 applies a transform, such as a discrete cosine transform (DCT) or a conceptually similar transform, to the residual block, producing a video block comprising residual transform coefficient values. Transform processing unit 52 may perform other transforms which are conceptually similar to DCT. Wavelet transforms, integer transforms, sub-band transforms or other types of transforms could also be used.

In any case, transform processing unit 52 applies the transform to the residual block, producing a block of residual transform coefficients. The transform may convert the residual information from a pixel value domain to a transform domain, such as a frequency domain. Transform processing unit 52 may send the resulting transform coefficients to quantization unit 54. Quantization unit 54 quantizes the transform coefficients to further reduce bit rate. The quantization process may reduce the bit depth associated with some or all of the coefficients. The degree of quantization may be modified by adjusting a quantization parameter. In some examples, quantization unit 54 may then perform a scan of the matrix including the quantized transform coefficients. Alternatively, entropy encoding unit 56 may perform the scan.

Following quantization, entropy encoding unit 56 entropy codes the quantized transform coefficients. For example, entropy encoding unit 56 may perform context adaptive variable length coding (CAVLC), context adaptive binary arithmetic coding (CABAC), syntax-based context-adaptive binary arithmetic coding (SBAC), probability interval partitioning entropy (PIPE) coding or another entropy coding technique. In the case of context-based entropy coding, context may be based on neighboring blocks. Following the entropy coding by entropy encoding unit 56, the encoded bitstream may be transmitted to another device (e.g., video decoder 30) or archived for later transmission or retrieval.

Entropy encoding unit 56 may use geometry information may be used for the determination of context information for CABAC coding. For example, when CABAC coding values for syntax elements of a block, entropy encoding unit 56 may determine one or more neighboring blocks to the block from which to retrieve information to be used to form the context information based on geometry information, e.g., distances between representative points of the block and the neighboring blocks. In some examples, entropy encoding unit 56 may weight the contributions of data from two or more neighboring blocks according to the geometry information, as discussed above.

Inverse quantization unit 58 and inverse transform unit 60 apply inverse quantization and inverse transformation, respectively, to reconstruct the residual block in the pixel domain, e.g., for later use as a reference block. Motion compensation unit 44 may calculate a reference block by adding the residual block to a predictive block of one of the frames of reference picture memory 64. Motion compensation unit 44 may also apply one or more interpolation filters to the reconstructed residual block to calculate sub-integer pixel values for use in motion estimation. Summer 62 adds the reconstructed residual block to the motion compensated prediction block produced by motion compensation unit 44 to produce a reconstructed video block for storage in reference picture memory 64. The reconstructed video block may be used by motion estimation unit 42 and motion compensation unit 44 as a reference block to inter-code a block in a subsequent video frame.

FIG. 16 is a block diagram illustrating an example of video decoder 30 that may be configured to perform the motion vector prediction techniques of this disclosure. In the example of FIG. 16, video decoder 30 includes an entropy decoding unit 70, motion compensation unit 72, intra prediction unit 74, inverse quantization unit 76, inverse transformation unit 78, reference picture memory 82 and summer 80. Video decoder 30 may, in some examples, perform a decoding pass generally reciprocal to the encoding pass described with respect to video encoder 20 (FIG. 15). Motion compensation unit 72 may generate prediction data based on motion vectors received from entropy decoding unit 70, while intra-prediction unit 74 may generate prediction data based on intra-prediction mode indicators received from entropy decoding unit 70.

During the decoding process, video decoder 30 receives an encoded video bitstream that represents video blocks of an encoded video slice and associated syntax elements from video encoder 20. Entropy decoding unit 70 of video decoder 30 entropy decodes the bitstream to generate quantized coefficients, motion vectors or intra-prediction mode indicators, and other syntax elements. In some examples, entropy decoding unit 70 may use the techniques of this disclosure to determine context information for entropy decoding values of syntax elements. For example, entropy decoding unit 70 may determine one or more neighboring blocks (and in some examples, weights) using geometry information (e.g., distances between a representative point of a current block and representative points of the neighboring blocks) to determine context information to be used to CABAC decode values of syntax elements for the current block. Entropy decoding unit 70 forwards the motion vectors and other syntax elements to motion compensation unit 72. Video decoder 30 may receive the syntax elements at the video slice level and/or the video block level.

When the video slice is coded as an intra-coded (I) slice, intra prediction unit 74 may generate prediction data for a video block of the current video slice based on a signaled intra prediction mode and data from previously decoded blocks of the current frame or picture. When the video frame is coded as an inter-coded (i.e., B, P or GPB) slice, motion compensation unit 72 produces predictive blocks for a video block of the current video slice based on the motion vectors and other syntax elements received from entropy decoding unit 70. The predictive blocks may be produced from one of the reference pictures within one of the reference picture lists. Video decoder 30 may construct the reference frame lists, List 0 and List 1, using default construction techniques based on reference pictures stored in reference picture memory 82.

Motion compensation unit 72 may form candidate lists, e.g., when decoding motion vectors using merge mode or AMVP mode, using geometry information between the current block and the neighboring blocks to determine the priority or insertion order for the construction of candidate lists such as a merge candidate list or an AMVP candidate list. Additionally or alternatively, intra prediction unit 74 may determine one or more most probable modes for intra prediction (where the most probable modes correspond to a candidate list) using geometry information between the current block and the neighboring blocks to determine the priority or insertion order for the most probable modes. In one example, the distance between a representative point of a current block and a representative point of the neighboring block is used as the geometry information to determine the priority or insertion order for the construction of the candidate lists. In one example, the shorter distance between that candidate's representative point and current representative point has higher priority, or vice versa. In another example, the distance can be LN-norm distance (N can be 1, 2 or any other positive integer).

Motion compensation unit 72 determines prediction information for a video block of the current video slice by parsing the motion vectors and other syntax elements, and uses the prediction information to produce the predictive blocks for the current video block being decoded. For example, motion compensation unit 72 uses some of the received syntax elements to determine a prediction mode (e.g., intra- or inter-prediction) used to code the video blocks of the video slice, an inter-prediction slice type (e.g., B slice, P slice, or GPB slice), construction information for one or more of the reference picture lists for the slice, motion vectors for each inter-encoded video block of the slice, inter-prediction status for each inter-coded video block of the slice, and other information to decode the video blocks in the current video slice.

Motion compensation unit 72 may also perform interpolation based on interpolation filters. Motion compensation unit 72 may use interpolation filters as used by video encoder 20 during encoding of the video blocks to calculate interpolated values for sub-integer pixels of reference blocks. In this case, motion compensation unit 72 may determine the interpolation filters used by video encoder 20 from the received syntax elements and use the interpolation filters to produce predictive blocks.

Video decoder 30 may be configured to perform any of the various techniques of this disclosure discussed above with respect to FIG. 14, and as will be discussed in more detail below. For example, motion compensation unit 72 may be configured to determine to perform motion vector prediction using AMVP or merge mode in accordance with the techniques of this disclosure. Entropy decoding unit 70 may decode one or more syntax elements representing how motion information is coded for the current block.

Assuming that the syntax elements indicate that merge mode is performed, motion compensation unit 72 may form a candidate list including a set of merge candidates. Motion compensation unit 72 may add candidates to the candidate list based on a particular, predetermined order. Motion compensation unit 72 may also add additional candidates and perform pruning of the candidate list, as discussed above. Ultimately, motion compensation unit 72 may decode a merge index representing which of the candidates is used to code motion information for the current block.

Inverse quantization unit 76 inverse quantizes, i.e., de-quantizes, quantized transform coefficients provided in the bitstream and entropy decoded by entropy decoding unit 70. The inverse quantization process may include use of a quantization parameter QP_(Y) calculated by video decoder 30 for each video block in the video slice to determine a degree of quantization and, likewise, a degree of inverse quantization that should be applied.

Inverse transform unit 78 applies an inverse transform, e.g., an inverse DCT, an inverse integer transform, or a conceptually similar inverse transform process, to the transform coefficients in order to produce residual blocks in the pixel domain.

After motion compensation unit 72 generates the predictive block for the current video block based on the motion vectors and other syntax elements, video decoder 30 forms a decoded video block by summing the residual blocks from inverse transform unit 78 with the corresponding predictive blocks generated by motion compensation unit 72. Summer 80 represents the component or components that perform this summation operation. If desired, a deblocking filter may also be applied to filter the decoded blocks in order to remove blockiness artifacts. Other loop filters (either in the coding loop or after the coding loop) may also be used to smooth pixel transitions, or otherwise improve the video quality. The decoded video blocks in a given frame or picture are then stored in reference picture memory 82, which stores reference pictures used for subsequent motion compensation. Reference picture memory 82 also stores decoded video for later presentation on a display device, such as display device 32 of FIG. 14.

FIG. 17 is a flowchart illustrating an example method of encoding video data according to the techniques of this disclosure. For purposes of example and explanation, the method of FIG. 17 is explained as being performed by video encoder 20 of FIG. 15. However, it should be understood that other devices may be configured to perform this or a similar method.

Initially, video encoder 20 receives a current block to be encoded (300). Mode select unit 40 then determines a prediction mode to be used to predict the current block (302). For example, mode select unit 40 may initially determine whether to use an intra-prediction mode or an inter-prediction mode. If mode select unit 40 determines to use intra-prediction, mode select unit 40 may further determine one of a variety of intra-prediction modes (e.g., a directional mode, DC mode, planar mode, or the like) to be used to predict the current block. If mode select unit 40 determines to use inter-prediction mode, motion estimation unit 42 may perform a motion search to determine a motion vector for one or more prediction units (PUs) of the current block.

In any case, video encoder 20 may predict the current block using the prediction mode (304). For example, in inter-prediction mode, motion compensation unit 44 may use the motion vector determined by motion estimation unit 42 to calculate a predicted block for the current block. As another example, in intra-prediction mode, intra-prediction unit 46 may generate the predicted block using values of neighboring pixels to the current block according to the determined intra-prediction mode.

Video encoder 20 may then determine distances between a representative point of the current block and representative points of neighboring blocks (306), such as spatial and/or temporal neighboring blocks. The representative points may correspond to, for example, center points of the blocks (e.g., as shown in FIGS. 12 and 13), or other representative points, such as top-left points of the blocks. The neighboring blocks may correspond to coding units, PUs, or sub-PUs, as discussed above.

Video encoder 20 then adds data from one or more of the neighboring blocks to a candidate list according to the determined distances (308). For example, for intra-prediction, video encoder 20 may determine a list of one or more most probable intra-prediction modes from the neighboring blocks, e.g., the intra-prediction modes used to predict the neighboring blocks having the shortest distances, where these blocks may be referred to as candidates included in the candidate list. As another example, for inter-prediction, video encoder 20 may form a candidate list of merge mode or AMVP mode for encoding a motion vector.

In any case, entropy encoding unit 56 may encode prediction information using the candidate list (310). For example, for intra-prediction, entropy encoding unit 56 may encode a syntax element representing whether, and which of, the most probable modes are used to predict the current block. If none of the most probable modes is used to predict the current block, entropy encoding unit 56 may further encode information representing which of a remaining set of intra-prediction modes is used to predict the current block. As another example, for inter-prediction, entropy encoding unit 56 may encode motion information according to merge mode or AMVP mode. For example, for merge mode, entropy encoding unit 56 may entropy encode an index into the candidate list. As another example, for AMVP mode, entropy encoding unit 56 may entropy encode an index into the candidate list, motion vector difference information, a reference picture list identifier, and an index into the reference picture list.

Video encoder 20 may also calculate a residual block for the current block (312). That is, as discussed above, summer 50 may calculate pixel-by-pixel differences between the predicted block and the original current block to calculate the residual block. Transform processing unit 52 may then transform the pixel differences of the residual block from the pixel domain (or spatial domain) to the frequency domain to produce transform coefficients, and quantization unit 54 may then quantize the transform coefficients, to thereby transform and quantize the residual block (314). Entropy encoding unit 56 may then entropy encode the quantized transform coefficients (316).

Although not shown in the example of FIG. 17, a method including steps similar to those of steps 306-310 may additionally or alternatively be used by entropy encoding unit 56 to entropy encode values for one or more syntax elements of a current block of video data. Such syntax elements may include, for example, any or all of a coding unit transquant bypass flag, a coding unit skip flag, a coded block flag, a prediction mode flag, a residual quadtree transform root coded block flag, a merge index, a merge flag, a coded block flag for a luminance block, or a coded block flag for a chrominance block. In general, such a method may include determining distances to neighboring blocks for the purpose of determining context information for CABAC coding. Entropy encoding unit 56 may use the context information to initialize and/or update a context model representing a probability of a bit of a binarized value having a value equal to a most probable symbol or a least probable symbol.

In this manner, the method of FIG. 17 represents an example of a method of coding video data including determining a plurality of distances between a first representative point of a current block of video data and a plurality of second representative points of neighboring blocks to the current block, adding one or more of the neighboring blocks as candidates to a candidate list of the current block in an order according to the distances between the first representative point and the second representative points, and coding (specifically, encoding, in this example) the current block using the candidate list.

FIG. 18 is a flowchart illustrating an example method of decoding video data in accordance with the techniques of this disclosure. For purposes of example and explanation, the method of FIG. 18 is explained as being performed by video decoder 30 of FIG. 16. However, it should be understood that other devices may be configured to perform this or a similar method.

Initially, in this example, video decoder 30 receives a current block to be decoded (330). The current block may be, for example, a coding unit (CU), a prediction unit (PU), a portion of a CU corresponding to a PU, a collection of sub-PUs, or the like. In accordance with the techniques of this disclosure, video decoder 30 determines geometry information, i.e., distances between a representative point of the current block and representative points of neighboring blocks to the current block (332). The representative points may be, for example, centers of the blocks, top-left corners of the blocks, or the like. The neighboring blocks may be PUs or sub-PUs, in some examples. Furthermore, the neighboring blocks may be spatial and/or temporal neighbors.

Video decoder 30 then adds data from the neighboring blocks to a candidate list according to the determined distances (334). The distances may generally represent priorities for ordering of the data in the candidate list, where shorter distances may generally represent higher priorities. As discussed above, the candidate list may be a candidate list of merge mode or AMVP mode decoding of a motion vector if the current block is inter-predicted, or a list of one or more most probable modes if the current block is intra-predicted. Video decoder 30 also determines the prediction mode (338), e.g., either inter- or intra-prediction, and predicts the block according to the prediction mode using the candidate list (340). In particular, if the prediction mode is intra-prediction, intra-prediction unit 74 determines an actual intra mode to be used to predict the block based on whether data indicates that one of the most probable modes is used, and if not, an identifier of the actual prediction mode. On the other hand, if the prediction mode is inter-prediction, motion compensation unit 72 may decode a motion vector for the current block according to merge mode or AMVP mode and generate a predicted block using the motion vector by retrieving data identified by the motion vector from reference picture memory 82.

Additionally, entropy decoding unit 70 entropy decodes quantized transform coefficients of the current block (342). Inverse quantization unit 76 inverse quantizes quantized transform coefficients for the current block, and inverse transform unit 78 applies an inverse transform to the transform coefficients, thereby inverse quantizing and inverse transforming the quantized transform coefficients (344), to produce a residual block. Summer 80 then adds values of the residual block with values of the predicted block on a pixel-by-pixel basis to decode the current block (346).

Again it should be understood that video decoder 30 may apply a method including steps similar to steps 332-336 of FIG. 18 when performing entropy decoding of values of various syntax elements, in some examples. Such syntax elements may include, for example, any or all of a coding unit transquant bypass flag, a coding unit skip flag, a coded block flag, a prediction mode flag, a residual quadtree transform root coded block flag, a merge index, a merge flag, a coded block flag for a luminance block, or a coded block flag for a chrominance block. In general, such a method may include determining distances to neighboring blocks for the purpose of determining context information for CABAC coding. Entropy decoding unit 70 may use the context information to initialize and/or update a context model representing a probability of a bit of a binarized value having a value equal to a most probable symbol or a least probable symbol.

In this manner, the method of FIG. 18 represents an example of a method of coding video data including determining a plurality of distances between a first representative point of a current block of video data and a plurality of second representative points of neighboring blocks to the current block, adding one or more of the neighboring blocks as candidates to a candidate list of the current block in an order according to the distances between the first representative point and the second representative points, and coding (specifically, decoding, in this example) the current block using the candidate list.

It is to be recognized that depending on the example, certain acts or events of any of the techniques described herein can be performed in a different sequence, may be added, merged, or left out altogether (e.g., not all described acts or events are necessary for the practice of the techniques). Moreover, in certain examples, acts or events may be performed concurrently, e.g., through multi-threaded processing, interrupt processing, or multiple processors, rather than sequentially.

In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media, or communication media including any medium that facilitates transfer of a computer program from one place to another, e.g., according to a communication protocol. In this manner, computer-readable media generally may correspond to (1) tangible computer-readable storage media which is non-transitory or (2) a communication medium such as a signal or carrier wave. Data storage media may be any available media that can be accessed by one or more computers or one or more processors to retrieve instructions, code and/or data structures for implementation of the techniques described in this disclosure. A computer program product may include a computer-readable medium.

By way of example, and not limitation, such computer-readable storage media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage, or other magnetic storage devices, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if instructions are transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. It should be understood, however, that computer-readable storage media and data storage media do not include connections, carrier waves, signals, or other transitory media, but are instead directed to non-transitory, tangible storage media. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc, where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media.

Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. In addition, in some aspects, the functionality described herein may be provided within dedicated hardware and/or software modules configured for encoding and decoding, or incorporated in a combined codec. Also, the techniques could be fully implemented in one or more circuits or logic elements.

The techniques of this disclosure may be implemented in a wide variety of devices or apparatuses, including a wireless handset, an integrated circuit (IC) or a set of ICs (e.g., a chip set). Various components, modules, or units are described in this disclosure to emphasize functional aspects of devices configured to perform the disclosed techniques, but do not necessarily require realization by different hardware units. Rather, as described above, various units may be combined in a codec hardware unit or provided by a collection of interoperative hardware units, including one or more processors as described above, in conjunction with suitable software and/or firmware.

Various examples have been described. These and other examples are within the scope of the following claims. 

What is claimed is:
 1. A method of coding video data, the method comprising: determining a plurality of distances between a first representative point of a current block of video data and a plurality of second representative points of neighboring blocks to the current block, the current block having a first size, at least one of the neighboring blocks having a second size that is different than the first size, the first representative point being smaller than the current block, and the plurality of second representative points being smaller than the corresponding neighboring blocks; determining a prediction mode for the current block; constructing a candidate list for the determined prediction mode for the current block, comprising adding motion information candidates representing one or more of the neighboring blocks to the candidate list in an order according to the plurality of distances between the first representative point and the plurality of second representative points, wherein codewords for candidate indexes of the candidates in the candidate list have sizes corresponding to positions of the candidates in the order; coding a codeword for one of the candidate indexes identifying one of the candidates in the candidate list; and coding the current block using the candidate in the candidate list identified by the one of the candidate indexes.
 2. The method of claim 1, wherein the candidate list comprises one of a merge candidate list, an advanced motion vector prediction (AMVP) candidate list, or an intra most probable mode (MPM) list.
 3. The method of claim 2, wherein the candidate list comprises the merge candidate list, and wherein coding the current block comprises coding motion information for the current block according to merge mode using the candidate of the merge candidate list.
 4. The method of claim 2, wherein the candidate list comprises the AMVP candidate list, and wherein coding the current block comprises coding motion information for the current block according to AMVP mode using the candidate of the AMVP candidate list.
 5. The method of claim 2, wherein the candidate list comprises the intra MPM list, and wherein coding the current block comprises coding an indication of an intra-prediction mode used to intra-predict the current block using the intra MPM list, and intra-predicting the current block using the intra-prediction mode.
 6. The method of claim 1, wherein at least one of the neighboring blocks comprises a sub-prediction unit (sub-PU), and wherein the one of the plurality of second representative points associated with the sub-PU comprises a center point of the sub-PU.
 7. The method of claim 1, wherein the first representative point of the current block comprises a center point of the current block and the plurality of second representative points of the neighboring blocks comprise respective center points of the neighboring blocks.
 8. The method of claim 1, wherein the first representative point of the current block comprises a top-left point of the current block and the plurality of second representative points of the neighboring blocks comprise respective top-left points of the neighboring blocks.
 9. The method of claim 1, wherein the neighboring blocks comprise one or more of spatially neighboring blocks to the current block or temporally neighboring blocks to the current block.
 10. The method of claim 1, wherein the candidate list comprises a list of candidates from which to determine context information for context-adaptive binary arithmetic coding (CABAC) of a value for a syntax element of the current block, and wherein coding the current block comprises CABAC coding the value for the syntax element of the current block using the context information determined from the list of candidates.
 11. The method of claim 10, wherein the syntax element comprises one of a coding unit transquant bypass flag, a coding unit skip flag, a coded block flag, a prediction mode flag, a residual quadtree transform root coded block flag, a merge index, a merge flag, a coded block flag for a luminance block, or a coded block flag for a chrominance block.
 12. The method of claim 10, further comprising determining the context information, comprising weighting contributions of values from the neighboring blocks according to the plurality of distances between the first representative point and the plurality of second representative points.
 13. The method of claim 1, wherein coding comprises encoding the current block using the candidate list.
 14. The method of claim 1, wherein coding comprises decoding the current block using the candidate list.
 15. A device for coding video data, the device comprising: a memory configured to store the video data; and one or more processors implemented in circuitry and configured to: determine a plurality of distances between a first representative point of a current block of video data and a plurality of second representative points of neighboring blocks to the current block, the current block having a first size, at least one of the neighboring blocks having a second size that is different than the first size, the first representative point being smaller than the current block, and the plurality of second representative points being smaller than the corresponding neighboring blocks; determine a prediction mode for the current block; construct a candidate list for the determined prediction mode for the current block, wherein to construct the candidate list, the one or more processors are configured to add motion information candidates representing one or more of the neighboring blocks to the candidate list in an order according to the plurality of distances between the first representative point and the plurality of second representative points, wherein codewords for candidate indexes of the candidates in the candidate list have sizes corresponding to positions of the candidates in the order; code a codeword for one of the candidate indexes identifying one of the candidates in the candidate list; and code the current block using the candidate in the candidate list identified by the one of the candidate indexes.
 16. The device of claim 15, wherein the candidate list comprises one of a merge candidate list, an advanced motion vector prediction (AMVP) candidate list, or an intra most probable mode (MPM) list.
 17. The device of claim 15, wherein at least one of the neighboring blocks comprises a sub-prediction unit (sub-PU), and wherein the one of the plurality of second representative points associated with the sub-PU comprises a center point of the sub-PU.
 18. The device of claim 15, wherein the first representative point of the current block comprises a center point of the current block and the plurality of second representative points of the neighboring blocks comprise respective center points of the neighboring blocks.
 19. The device of claim 15, wherein the candidate list comprises a list of candidates from which to determine context information for context-adaptive binary arithmetic coding (CABAC) of a value for a syntax element of the current block, and wherein the one or more processors are configured to CABAC code the value for the syntax element of the current block using the context information determined from the list of candidates.
 20. The device of claim 15, wherein the device comprises at least one of a video encoder configured to encode the current block, a video decoder configured to decode the current block, or a wireless communication device.
 21. A device for coding video data, the device comprising: means for determining a plurality of distances between a first representative point of a current block of video data and a plurality of second representative points of neighboring blocks to the current block, the current block having a first size, at least one of the neighboring blocks having a second size that is different than the first size, the first representative point being smaller than the current block, and the plurality of second representative points being smaller than the corresponding neighboring blocks; means for determining a prediction mode for the current block; means for constructing a candidate list for the determined prediction mode for the current block, comprising means for adding motion information candidates representing one or more of the neighboring blocks to the candidate list in an order according to the plurality of distances between the first representative point and the plurality of second representative points, wherein codewords for candidate indexes of the candidates in the candidate list have sizes corresponding to positions of the candidates in the order; means for coding a codeword for one of the candidate indexes identifying one of the candidates in the candidate list; and means for coding the current block using the candidate in the candidate list identified by the one of the candidate indexes.
 22. The device of claim 21, wherein the candidate list comprises one of a merge candidate list, an advanced motion vector prediction (AMVP) candidate list, or an intra most probable mode (MPM) list.
 23. The device of claim 21, wherein at least one of the neighboring blocks comprises a sub-prediction unit (sub-PU), and wherein the one of the plurality of second representative points associated with the sub-PU comprises a center point of the sub-PU.
 24. The device of claim 21, wherein the first representative point of the current block comprises a center point of the current block and the plurality of second representative points of the neighboring blocks comprise respective center points of the neighboring blocks.
 25. The device of claim 21, wherein the candidate list comprises a list of candidates from which to determine context information for context-adaptive binary arithmetic coding (CABAC) of a value for a syntax element of the current block, and wherein the means for coding the current block comprise means for CABAC coding the value for the syntax element of the current block using the context information determined from the list of candidates.
 26. A non-transitory computer-readable storage medium having stored thereon instructions that, when executed, cause a processor to: determine a plurality of distances between a first representative point of a current block of video data and a plurality of second representative points of neighboring blocks to the current block, the current block having a first size, at least one of the neighboring blocks having a second size that is different than the first size, the first representative point being smaller than the current block, and the plurality of second representative points being smaller than the corresponding neighboring blocks; determine a prediction mode for the current block; construct a candidate list for the determined prediction mode for the current block, comprising instructions that cause the processor to add motion information candidates representing one or more of the neighboring blocks to the candidate list in an order according to the plurality of distances between the first representative point and the plurality of second representative points, wherein codewords for candidate indexes of the candidates in the candidate list have sizes corresponding to positions of the candidates in the order; code a codeword for one of the candidate indexes identifying one of the candidates in the candidate list; and code the current block using the candidate in the candidate list identified by the one of the candidate indexes.
 27. The non-transitory computer-readable storage medium of claim 26, wherein the candidate list comprises one of a merge candidate list, an advanced motion vector prediction (AMVP) candidate list, or an intra most probable mode (MPM) list.
 28. The non-transitory computer-readable storage medium of claim 26, wherein at least one of the neighboring blocks comprises a sub-prediction unit (sub-PU), and wherein the one of the plurality of second representative points associated with the sub-PU comprises a center point of the sub-PU.
 29. The non-transitory computer-readable storage medium of claim 26, wherein the first representative point of the current block comprises a center point of the current block and the plurality of second representative points of the neighboring blocks comprise respective center points of the neighboring blocks.
 30. The non-transitory computer-readable storage medium of claim 26, wherein the candidate list comprises a list of candidates from which to determine context information for context-adaptive binary arithmetic coding (CABAC) of a value for a syntax element of the current block, and wherein the instructions that cause the processor to code the current block comprise instructions that cause the processor to CABAC code the value for the syntax element of the current block using the context information determined from the list of candidates. 